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11.7 Mental retardation: public health approaches to intellectual impairment and its consequences

11.7 Mental retardation: public health approaches to intellectual impairment and its consequences
Oxford Textbook of Public Health

Mental retardation: public health approaches to intellectual impairment and its consequences

Tom Fryers

Concepts, classifications, terms, and labels

Current classifications

Global categories

Partial categories

Descriptive epidemiology: frequencies in populations

Factors affecting frequencies

Severe intellectual impairment (IQ < 50)

Mild intellectual impairment (IQ 50–69)

Mild learning disability and mild dependency related to intellectual impairment
Central nervous system impairment: aetiological diagnosis and prevention of intellectual impairment

Types and frequencies of organic syndromes

Cause, frequency, and prevention

Specific aetiological groups: causal processes, treatment, and prevention of the most important syndromes

Commonly associated syndromes of impairments and/or disabilities
Specific disabilities

Identification and assessment of mentally retarded people
Dependency: community needs and organization of services

History, organization, and philosophy of care

A revolution in care

Developing countries

Components of community-based mental retardation services
Conclusion and summary
Useful addresses



Specific syndromes, neurological impairments, and prevention

Disabilities and their consequences: identification and assessment

Overview of service provision: philosophy, law, planning, evaluation

Elements of care
Chapter References

The public health tradition arose from the need to protect the public from communicable disease. This required study of the epidemiology; a preventive approach to disease control which encompassed specific preventive measures, early diagnosis, and prompt, effective treatment; and a planned community overview of the provision of services.
Even for non-communicable disease, disorder, and disability, men and women living in the community are interdependent, and serious disadvantage in some members affects all of us in a broad commonality of health, disease, and disorder. A humane and compassionate society acknowledges this and our common responsibility, wherever possible, to improve health, to prevent, treat, and ameliorate disease and its consequences, to care for those most damaged and disadvantaged, and to share the burden which falls upon them. In the field of mental retardation, the burden to be shared falls not only upon the affected individuals, but also upon their immediate families to a disproportionate degree.
Therefore the three principle elements of public health concern are epidemiological elucidation of the nature and size of the problem, promotion of prevention, and provision of community-wide programmes of habilitation and care. Epidemiology will include searching for causes, studying the frequency of impairments, disabilities, and their consequences in different communities and at different times, and evaluating services. Preventive programmes offer many possibilities. Primary prevention can be implemented through mass screening, immunization, or control of the physical environment. Early assessment, and monitored programmes of habilitation help to facilitate development and minimize secondary disabilities. Social consequences can be reduced by organized support for the individual and the family, ameliorating adverse social conditions, and challenging stigmatizing attitudes and behaviours in the social environment. To plan community-wide habilitation and care requires an understanding of communities and cultures as well as the biology of intellectual impairment and its associated phenomena. A key problem will be co-ordination of many agencies and personnel.
Attitudes have changed a great deal in the past 30 years. Our inherited systems of care in industrialized countries were mostly based on large, remote, custodial, residential institutions, dominated by doctors, and capital intensive. These are alien to the basic philosophies of normalization, integration, human rights, and client choice which inform a modern public health approach to the disabled, the elderly, and the long-term sick. Programmes are necessarily multidisciplinary and labour intensive. The transformation of services has already proceeded a long way in many countries, not always adequately resourced or well managed, and not without problems, but changes on the scale required cannot be brought about overnight, or without commitment, conflict, and cost. They bring problems of a high order for the public health physician and his or her colleagues, which are amongst the most challenging on the current public health scene.
Concepts, classifications, terms, and labels
Mental retardation is neither a single entity nor a medical diagnosis, and the field is beset by serious problems of conceptualization, classification, and terminology. Promotion of normalization has led some to question the ethics of classifying people in terms of their disease, disorder, or disability because of the social consequences of ‘labelling’. This concern is entirely proper, but planning requires categories, and scientific measurement requires clear definitions and a logical taxonomy. Moreover, however desirable the integration of disabled people into ordinary society, if they disappear from view they are likely to be ignored in the provision of the special help they need.
Disabled people do have special needs by definition. They cannot compete on equal terms with their fellows, and are unlikely, without help, to escape the low status and stigma which tends to be their lot. Normalization must not excuse invisibility and neglect, leaving the burden of care entirely on the disabled person and their immediate family. Epidemiology can clarify thinking about disabled people and their needs, and inform the planning of services and the allocation of resources.
For over a century, concepts and terms related to mental retardation have been debated and disputed. In earlier years, dominant concepts usually implied immutable pathologies, present at birth and largely inherited. More recently favoured concepts involve disrupted development caused by a variety of interacting physical and social determinants, with potential for further development. Underlying all of them is a common perception of people at the bottom of the scale of human competence who cannot survive independently.
But what are the criteria for survival in human society? Mental retardation engages the interests of paediatricians, geneticists, biochemists, psychologists, psychiatrists, educationalists, managers, social legislators, and others. Each group sees the client group, its needs, and problems differently, and each has promulgated its own definitions based on its own concepts. These are inherently difficult to reconcile: poor genetic potential, low measured intelligence, insufficiency in learned competencies, inadequate adaptation to society, social dependency, multiple physical and psychological disabilities, central nervous system damage or disorder, and specific pathological or aetiological entities.
Arising from this diversity of professional and scientific perspective, the literature of mental retardation provides a remarkable variety of terms, inconsistency of categories, and ambiguity of concepts. And for all these, measurement is problematic, especially in children, in whom we are implicitly trying to predict how they will function as adults. We must use age-related norms of function and behaviour which are not easily standardized across cultures or rendered precise in measurement. There are also difficulties of degree, since divisions are arbitrary, and milder expressions of developmental curtailment raise essentially different practical issues as they merge into ‘normal’. In practice, most reported studies are small scale and pragmatic; they seldom discuss taxonomy or define the terms and categories used. Thus much of the literature is useless for comparison or application elsewhere.
Current classifications
The classification of mental retardation in the International Classification of Diseases, 10th Revision (ICD-10) (WHO 1992a) is simple but inadequate, depending upon intelligence quotient (IQ) criteria rarely available in practice. The classifications produced by the American Association on Mental Retardation (AAMR 1992) based upon their Adaptive Behaviour Scales (Grossman 1983) and in the Diagnostic and Statistical Manual of Mental Disorders (Fourth Edition) (DSM-IV) (APA 1980) are fairly closely related, using IQ data with qualifying criteria. The former is more precise, detailed, and standardized. Both tend to mingle concepts of impaired structure, disordered function, and social disadvantage, the discrimination of which is the great strength of the International Classification of Impairments, Disabilities and Handicaps (ICIDH) (WHO 1980). This conceptualization offers a useful framework for a general taxonomy for mental retardation, using the following definitions: ‘impairment’ is any loss or abnormality of psychological, physiological, or anatomical structure or function; ‘disability’ is any limitation of capacity to perform an activity in the manner or within the range considered normal for any human being; ‘handicap’ is any disadvantage which an individual experiences in the fulfilment of a social role that is considered normal for a person of his or her age and gender in that culture because of an impairment or disability.
The WHO classification has undergone revision (WHO 2001), changing both the concept boundaries and the terms used. ‘Impairment’ is retained but includes all loss of function below activities at the whole person level. The term ‘disability’ is used as an umbrella term for all imperfections and is replaced at the whole person level by ‘limitation of activity’. ‘Handicap’ is replaced by ‘restriction of participation’ with a more overt emphasis on society’s contribution to the experience of disadvantage. These changes were finally published in January 2001.
Other usages will continue and we cannot expect the new terms to be any less problematic than the old. Socially, an acceptable group ‘label’ may change to recognize changes in social perceptions, and to keep a step ahead of stigma. For example, in the United Kingdom, it has been ‘mental deficiency’, ‘mental subnormality’, and ‘mental handicap’ over the last 40 years, and is currently ‘learning disabilities’. No doubt it will change again in the future. Other English-speaking countries use ‘mental impairment’, ‘intellectual disability’, ‘developmental disability’, ‘learning handicap’, and many others. The acceptability of terms to parents is as important as scientific study but does not necessarily follow the same logic, and scientific and social purposes differ. We have suffered very unscientific confusion, lack of compatibility, and waste of effort by professionals and researchers adopting whatever title is currently in use in their own community, assuming that definitions remain the same, and implying that there is no necessity for a standardized scientific taxonomy and terminology!
Lay and scientific use of terms should not be confused. Scientific categories are primarily for research into human groups. They can be applied to service planning, monitoring, and evaluation with care, but should be applied to individuals in service contexts only with great caution. To determine individual needs, individual assessment is required; it should not depend upon their allocation to a group or their acquisition of a group label. In this chapter, ‘mental retardation’ is used as the general title for the domain of scientific and professional activity, because it has long international usage and does not conflict with the ICIDH. Where applied (on behalf of all those other terms used in particular societies) to a group of people, it represents a category of social disadvantage. The terms used below, while based upon ICIDH concepts, are intended also to preclude conflict with ICIDH-2 in the future. To this end, ‘handicap’, the most contentious of the previous terms, has been avoided.
We should first distinguish between ‘global’ and ‘partial’ categories (Table 1). Global categories are those which purport to describe the client group as an entity entirely within the field of mental retardation, from one specified point of view or another. They imply a global or generalized impairment, disability, or social disadvantage (handicap) and to avoid ambiguity, ‘generalized’ is used in each case. Partial categories are those which are not exclusively related to or defined in terms of mental retardation, and thus encompass both individuals who are considered mentally retarded and those who are not. In principle there is no limit to the number of global or partial categories, but three of the former and five of the latter may be of most practical use (Fryers 1993).

Table 1 Taxonomies in mental retardation

Global categories
Generalized intellectual impairment
Intelligence—the capacity to learn, to apply learning, and to develop—can be operationalized broadly to include all aspects of learning, or narrowly to specify only one aspect. The organic basis of this capacity in the central nervous system, although not fully understood, may be conceived as a body system, called ‘the intellect’ analogous to biochemical systems to which the ICIDH applies the term ‘impairment’.
Definitions are inextricably bound up with the means of measurement. Intelligence tests, the results of some of which can be summarized as an IQ, are far from consistent, do not entirely disentangle learned skills from underlying capacities, and can be misused, but nevertheless have validity and usefulness (Berger and Yule 1985). For individuals, they can provide a useful indication of current development if they are professionally administered and include subscores and commentary, but their predictive value is limited. Many children show increments of IQ as they mature, probably recovering from early lack of stimulation. Intelligence as measured is undoubtedly influenced by both nature and nurture, and determinants of impairment include general polygenic inheritance, specific genetic anomalies, organic damage, lack of education, emotional state, and social deprivation. Some of these are ameliorable.
IQs are entirely comparative, and a product of populations, showing essentially a statistically ‘normal’ distribution, but test means in particular populations change over time, subpopulations differ, and there is an excess at the lower end of the range due to brain damage and disorder. Any IQ band can be studied in its associations with any other characteristic of people in that population, their experience, or their environment. It is not unlike studying height, although IQs have no reference values outside human populations. People with very low IQs, like people with very low heights, are likely to have problems in society, and it is important to study their characteristics and needs.
Although particular IQ categories are arbitrary, the conventions of IQ < 50 for ‘severe intellectual impairment’ (IQ < 55 would have equal validity, but has been used much less in epidemiological research) and IQ 50–69 for ‘mild intellectual impairment’ carry considerable weight, especially the former. Subdivisions in the ICD and ICIDH of the IQ < 50 category into ‘profound’, ‘severe’, and ‘moderate’ have not proved very useful, as IQ measures below 35 are much less reliable (Fryers 1984, 1993). All people of IQ < 50, in all societies, will be considered learning disabled and mentally retarded, so IQ is the sole essential criterion. For IQ 50–69, additional criteria are always used to define the group in society. Mild mental retardation is never the same as mild intellectual impairment.
Generalized learning disability (related to intellectual impairment)
Where global impairment is of the intellect, global disability is of the function of learning, encompassing acquisition of knowledge, skills, and attitudes, mental development, and personality maturation. As commonly used in educational systems, specific categories of learning disability rarely match IQ categories. There are many reasons for children to be identified as having learning disabilities; current usage in the United Kingdom usually links these to a substantial degree of intellectual impairment, but American usage does not. If the criteria for selection and the measures used are stated, the relationship will be clear. All children of IQ < 50, i.e. those who are severely intellectually impaired, will always be identified as severely learning disabled, but children in the mildly intellectually impaired group may not be identified as learning disabled in some school contexts.
Determinants of generalized learning disability in populations may include, as well as intellectual impairment, motor and sense organ impairments, social and educational deprivation, childhood psychosis, highly selective school systems, inadequate teachers, administrative conventions, and legal requirements. Standard scientific categories are not available. Learning disability should be measured by tests of learning function, but tests usually provide proxy measures of the outcome of the learning process by testing performance of skills such as memorization and recall, reading, writing, and numeracy, and relating them to age and culture norms. It remains problematic to summate any collection of these as generalized learning disability.
For adults, measures of social maturity as in the Vineland Social Maturity Scale, and behavioural age, as derived from the American Association of Mental Deficiency Adaptive Behaviour Scales may also offer proxies for generalized learning disability. They have been much used and are standardized, although they should be applied to different populations with caution. Categories are independent of IQ, so offer a dimension which can be studied in relation to intellectual impairment. It is important to remember that there are groups of adults considered to have learning disabilities not closely related to intellectual impairment, especially prison inmates (BSU 1994).
Generalized dependency (related to intellectual impairment)
Social disadvantage associated with any impairment or disability is a very personal experience in a particular social context. Collectively, the principle general social disadvantage related to diminished intelligence shared by all those identified as mentally retarded (whatever the label) is lack of capacity for independence, or ‘dependency’. This can be viewed as a summation of the ‘major survival roles’ in ICIDH. It offers six examples—orientation, physical independence, mobility, occupation, social integration, and economic self-sufficiency—but many more could be described.
Like learning disability, criteria for and measures of dependency need specifying. In the context of mental retardation, it is ‘related to’ intellectual impairment, but IQ has never been applied as the sole criterion. Prevalence differentials according to sex, social class, and race, and excess frequencies of sensory and motor deficits, psychiatric disturbance, challenging behaviour, and family disruption suggest that many social criteria are also applied.
It is these selection criteria—some overt, some covert—which determine who is perceived, categorized, registered, treated, or served as ‘mentally retarded’ in any one community. They will vary substantially from community to community, but are generally those which bring clients into contact with certain educational, medical and social services, or the law. Low intelligence—measured, guessed, or assumed—is usually, but not always, one overt factor, as are certain obvious medical diagnoses such as Down’s syndrome. But many selection criteria are not overt, reflecting education, mental health, welfare and criminal law, and juridical practice; organizational characteristics of the services; professional conventions and perceptions; and government policies and public attitudes (Table 4). As Haywood (1970) wrote, ‘Understanding the causes of handicap involves understanding the social system of which handicapped children are a marginal component.’

Table 4 Estimates of prevalence of severe intellectual impairment (IQ < 50) for a standard English district of stable population

Selection, therefore, depends as much on the characteristics of services and society as of clients, but few studies have examined these to identify the selection processes adopted and the underlying social attitudes and societal context which created them. As human services emphasize normalization and integration of people with disabilities, such studies could be immensely useful.
A group of people with generalized dependency is identified and labelled, no doubt, in every society. If IQ < 50 is a criterion, severe intellectual impairment is being defined, but all such people will experience severe dependency too. This is not true for mild intellectual impairment; very few people of IQ 50–69 are selected as mentally retarded, and those who are usually exhibit challenging behaviour, mental illness, multiple physical disabilities, or contact with the criminal law. In some communities, people with similar problems but higher IQs may also be included.
Because mental retardation—generalized dependency related to intellectual impairment—is socially determined and very variable in population frequency (usually called ‘administrative’ prevalence), researchers have sometimes concluded that comparison is impossible but this is not so. Groups defined by IQ < 50 we can compare as severe intellectual impairment (Fryers 1984). Otherwise we must study the factors which determine selection and identification as ‘mentally retarded’ in different societies, the social processes involved, and the benefits and ‘dis-benefits’ for clients.
Partial categories
Persons with intellectual impairment may experience any disease or disorder, but some are commonly associated with intellectual impairment or commonly found in those selected as mentally retarded, and may be regarded as categories partially related to mental retardation. Serious investigation of causes and the potential for prevention requires studies of total human populations across all intelligence levels, not only of individuals identified as mentally retarded. Studies of the proportional composition of groups identified as mentally retarded, as regards aetiological diagnosis for example, are of limited value.
Any number of types of partial category could be described, but five types may be most useful as follows.
Physical impairments: aetiological and pathological groups
These mostly relate to neurological impairments. They are rarely limited to specific degrees of intellectual impairment or learning disability and many include people who would never be considered mentally retarded. Examples are phenylketonuria (including treated cases), Down’s syndrome (including mosaics), and fragile X (including female carriers). Some common organ impairments are part of recognized syndromes also including diminished intelligence, such as rubella syndrome or fetal alcohol syndrome. Congenital cardiac defects and Alzheimer’s dementia are found more commonly in Down’s syndrome than the rest of the population. Congenital abnormalities tend to associate with each other in populations; trauma tends to produce multiple effects in individuals.
Syndromes of impairments and/or disabilities
These include the epilepsies, cerebral palsies, psychiatric disorders, and autistic spectrum disorders (‘pervasive developmental disorders’); ‘challenging behaviour’ may be included here, although syndromes are less well defined. They are neither aetiological nor pathological entities, but are important in relation to mental retardation, showing higher frequencies than the general population, and posing serious problems for clinical and psychological assessment. The epidemiology of each of these groups of disorders is complex and difficult, and constantly interacts with that of intellectual impairment and mental retardation.
Specific disabilities: losses of function
Motor and sensory disabilities are common in people with severe intellectual impairment, and represent additional reasons for selection as mentally retarded in the presence of mild intellectual impairment. Similar losses arise in diverse ways, so disability categories do not correlate with IQ categories, organic impairments, or aetiological entities. Nor do similar disabilities always produce similar social consequences. Research demands a broad approach. Specific disabilities such as ‘mobility’ or ‘inability to feed oneself’ have been increasingly studied in recent years, and more guidance for habilitative practice is likely to emerge in the future. Some degree of standardization of measures is emerging (Fryers 1993). The ‘academic skills disorders’ of DSM-IV may be seen as specific learning disabilities related partially to intellectual impairment.
Social consequences: individual disadvantages
Specific disadvantages are poorly defined and little researched, although there is great potential in such studies for improving the lives of people identified as mentally retarded. Many social consequences of intellectual impairment are also experienced by other disabled people, the poor, and the disenfranchised. The social consequences depend upon the constellation of impairments and disabilities, abilities and aspirations, individual temperament and attitudes, and the demands, opportunities, and resources of the family and society in which he or she lives.
Some disadvantages, such as in housing, employment, and welfare benefits, have been much studied in general, but seldom including people with intellectual impairment. Few studies have examined marriage, friendship, social networks, employment, and leisure opportunities; a notable exception is the Aberdeen study following up all identified mentally retarded children in that Scottish city to observe a wide range of adult life patterns, opportunities and disadvantages (Koller et al. 1988; Richardson et al. 1988, 1993). More research is needed; it is social disadvantages which provoke the greatest frustrations and the greatest sense of alienation. The principle aim of services for people with intellectual impairment is the reduction of these disadvantages. Rarely can impairment be diminished; sometimes disability can be ameliorated; but most improvement in people’s lives will come from addressing disadvantages directly through enhanced opportunities, extra assistance, positive discrimination, and political action.
Carers’ concerns: parent and family disadvantages
The demands on parents caring for a severely intellectually impaired child has long been acknowledged, but many aspects are little researched (Twigg 1992; Beresford 1993; Keltner and Ramey 1993). As institutional care diminishes, more responsibility tends to fall on parents, siblings, and other family members, for adults who now frequently survive their parents. Community service agencies and professional staff are dependent upon such informal ‘carers’. They should be considered partners in care programmes, but carers’ own needs should not be ignored. Social scientists are only just beginning to address these issues.
Most classification systems have used a combination of IQ, educational failure, social adaptation, and medical diagnosis without precise relationships being specified. Imprecise and non-standard measures have increased the ambiguity. No single simple set of categories will suffice for all purposes within the field of mental retardation, but the basic concepts of the ICIDH offer multiple dimensions within a unified structure which resolves many of the difficulties.
Mental retardation probably remains the best general title for the field of study, with three sets of global categories—generalized intellectual impairment, generalized learning disability, and generalized dependency. These can be clearly defined by measurable criteria, and studied in relation to one another. Several sets of partial categories describe groups not exclusive to mental retardation and encompassing aetiological and pathological entities, clinical syndromes, specific disabilities, and social consequences for both individuals and carers. Causes of impairment, disability, and disadvantage, however defined, will be found in society as much as in individuals.
Research workers need to be wary and clear thinking; ambiguous definitions and imprecise measures waste time and resources. Service planners and managers need to understand the conceptual problems before applying simple criteria to their service situations. It is important that no classification system, however elegant to the epidemiologist or convenient to the manager, should be allowed to determine lifetime services for an individual! Lifetime institutionalization was the nightmare of labelling. Taxonomy is concerned with groups; individuals require thorough, multidisciplinary, professional assessment, constantly monitored and updated, and sensitively involving the client and his or her family where appropriate. This should help to avoid the problems of labelling, administrative determinism, institutional batch management and inflexible service structures, and ensure the best practical attention to constantly changing individual needs.
Descriptive epidemiology: frequencies in populations
Factors affecting frequencies
Incidence and prevalence of a heterogeneous category like mental retardation are highly dynamic. However defined, they are determined by the frequency of occurrence of very many disorders of widely varying genesis. Some disorders arise at conception and their causes must be looked for before conception; others arise in early fetal life, around birth, and in early postnatal life. New cases become progressively less frequent with age, and cumulative inception almost levels off by 4 or 5 years. But mortality of abnormal fetuses follows a similar dynamic, being concentrated in early fetal life, around birth and in early infancy, then almost levelling off. Prevalence at any age is determined by both inception rates and mortality rates before that age, although known prevalence also depends upon the identification rate (Fig. 1) Prevalence ratios are also susceptible to differential migration which changes the relationship between numerators and population denominators.

Fig. 1 Schematic diagram of the relationship between aetiology, mortality, and ascertainment in severe intellectual impairment. The two vertical lines represent conception and birth, respectively. The upper curve represents the cumulative incidence of disorders associated with severe intellectual impairment in a notional cohort of conceptuses. At conception, many individuals are already impaired; more arise early in fetal life, fewer later on. The perinatal period results in a relatively large increase in numbers, but incidence thereafter progressively diminishes. The lower curve represents the cumulative mortality of affected individuals within the cohort. Many impaired fetuses die early in fetal life, fewer later on. The perinatal period carries higher risks, but after birth mortality progressively diminishes. The vertical distance between the two curves represents the ‘true prevalence’ at any given age. Not all those impaired at birth are recognized at the time; ascertainment is progressively achieved, as represented by the dotted curve. The ‘known prevalence’ at any given age is represented by the vertical distance between the ascertainment curve and the mortality curve.

‘Incidence’ carries little meaning for the group as a whole, and ‘true incidence’ is difficult to establish even for individual syndromes. In Down’s syndrome it must take account of the large proportion of naturally aborted fetuses, not easily estimated. In perinatal aetiologies it must encompass all similar adverse factors, whether or not the outcomes include intellectual impairment. The incidence of affected births varies with the current local population birth rate, so ‘birth frequency’ (or ‘prevalence at birth’: the proportion of cases in a continuous series of births in a given population) is used. Even this is difficult to know except for well-defined syndromes easily recognized at birth.
The dynamic processes of inception, mortality, and migration give rise to substantial variation in prevalence in different communities and at different times. There is substantial reliable research literature only with respect to groups defined by IQ < 50 (severe intellectual impairment, but usually called severe mental retardation, and so on). It reveals general characteristics summarized in Table 2 (Fryers 1984). The common assertion that prevalence ratios for severe mental retardation are ‘stable’ or consistent is not true, even for clearly defined severe intellectual impairment. It would, indeed, be very strange, given the social determinants of population frequency of the causes of neurological impairment, which vary so much between cultures and communities. These social factors (e.g. diet in neural tube defects; patterns of fertility in Down’s syndrome; alcohol price and supply in fetal alcohol syndrome; consanguinity in recessive genetic disorders) are most likely to offer scope for prevention.

Table 2 Basic epidemiology of severe intellectual impairment: prevalence in developed communities

Severe intellectual impairment (IQ < 50)
This is coterminous with severe learning disability, mental retardation, or any other group which is defined as IQ < 50.

Point prevalence varies between similar birth cohorts (concurrent age groups) in different communities. Reliable studies found 1.62 per 1000 children born between 1951 and 1955 in Salford, United Kingdom, and 7.34 per 1000 children born in 1957 in Amsterdam, The Netherlands. Greater variation is expected in developing countries. Prevalence varies with genetic, cultural, economic, environmental, and service factors, largely by influencing the spectrum of biomedical causes and early mortality. Sometimes one cause dominates the scene, especially iodine deficiency disease, where more than 10 per cent of village populations can be affected by endemic congenital hypothyroidism (‘cretinism’). In developed countries, Down’s syndrome may be the most prevalent aetiological group in children, especially in communities with traditions of late marriage, large families, and taboos against contraception and/or abortion. But where early general mortality is high, few infants with Down’s syndrome survive into later childhood. Congenital anomalies are relatively high in communities with traditions of consanguinous marriage, but few are associated with intellectual impairment. Mortality and survival are extremely variable often related to the ‘development’ status of the community.

Age-specific prevalence changes over time in the same community, because the spectrum of causes and the effects of early mortality change. In Salford, prevalence for children aged 5 to 9 years was 1.98 per 1000 in 1961, 5.54 per 1000 in 1971, and 3.86 per 1000 in 1980. The factors which explain differences between communities may explain changes over time in the same community.

A similar pattern of temporal change is seen throughout most of the developed world, with a low range of values for age-specific prevalence (1.8–4.0 per 1000) for children born in the early 1950s, a high range (3.3–5.5 per 1000) for those born in the mid-1960s, and falling prevalence ratios since then, at least well into the 1980s (Rumeau-Rouquette et al. 1997). There are exceptions: high recent frequencies in Northern Ireland, for example, are due to special cultural circumstances. In Fig. 2, reliable prevalence data for small age groups of children from many studies are plotted according to their period of birth to compare them as cohorts.

Fig. 2 Severe intellectual impairment (IQ < 50) from published studies in developed countries: age-specific prevalence ratios in school-age children related to years of birth. Each line represents the prevalence and years of birth for one age group of children in each study. The continuous line represents changing annual prevalence applied to years of birth, for 5–9- and 10–14-year-old children in Salford, United Kingdom, through 25 years (Fryers 1984, 1991).

Increased prevalence can be related to a rapid decrease in early mortality associated with developments in neonatal care. This increased survival is well documented for Down’s syndrome by life-table studies from birth (Table 3). Other aetiological groups are too rare or inconsistent in case definition to be studied in a similar way, but we can assume similar processes continue to increase survival.

Table 3 Changes in survival in Down’s syndrome: life tables, 1940–1980

The progressive decrease in prevalence in young children since the late 1960s reflects many different processes reducing inceptions. Large-scale oral contraception reduced both mean maternal age and, especially, conceptions in older women, which greatly reduced the incidence of Down’s syndrome. In the 1980s, amniocentesis and abortion programmes began to reduce Down’s syndrome even more, although the impact was diminished by the lack of conceptions in older women. In the 1990s, the average age of conception has tended to increase again. This, together with new screening techniques applied at all maternal ages, increases the potential impact of abortion programmes.
Other contributions are very small because each aetiological group is small, but the cumulative effect may be significant. Postnatal screening programmes for inherited metabolic disorders and sporadic congenital hypothyroidism are very successful. Although conclusions are difficult to draw from the evidence, perinatal factors probably increased the number of babies with neurological impairments surviving in the 1970s but decreased them in the 1980s.
The reduction of encephalitis, encephalopathy, and rubella syndrome by effective immunization (and other measures for tuberculosis and bacterial meningitis) probably had a small effect in the 1970s and 1980s. However, a major impact can be anticipated in the 1990s in countries that achieve immunization rates sufficient to produce herd immunity for pertussis, measles, mumps, Haemophilus influenzae type B, and rubella.
The widespread adoption of early stimulation and training programmes for severely intellectually impaired infants, especially those with Down’s syndrome may have removed a few individuals from the ‘severe’ group at later ages by improving their test performance, but the impact on prevalence rates would be extremely small and experienced only once in one community.

Prevalence varies by age. Age distributions change over time because of differences in birth cohorts and survival from early infancy. Although not ‘in phase’ everywhere, in many developed countries the highest age-specific prevalence is for those born in the early to mid-1960s. Thus, by 2000 the largest age group were approaching their mid-thirties. Younger age groups (i.e. later birth cohorts) have had progressively lower prevalence ratios. This has meant progressively fewer children throughout the last 20 years. The larger cohorts will, of course, move up the age range in future decades.
Prevalence also varies by age because reduced mortality has increased survival at all ages (McGrother et al. 2001). In most developed countries there are currently more adults over 45 than children under 15 years of age, and there are substantial numbers of elderly severely intellectually impaired people. Professionals and managers needs to recognize this.

From prevalence and mortality data, estimates can be made of age-specific prevalence ratios for severe intellectual impairment (IQ < 50) in developed countries. They assume a stable population over the last half century and no unusual factors affecting particular important aetiological groups. Figures for the youngest and oldest age groups are informed guesses. In Table 4, projections for a ‘standard’ United Kingdom District for 1.1.00 are more speculative than the original estimates for 1.1.90 because reliable recent data are few. The few recent studies are mostly consistent with them, and they should be useful guides for planning and resourcing community-based services, modifying the figures to take account of migration and other particular features of a real district.

There are usually more males than females at all ages, the ratio varying between 1 in 1 and 2 in 1, but with no clear pattern. It probably depends upon the particular spectrum of causes of central neurological damage and disorder in any population (some biomedical causes, such as fragile X, favour males), but differential mortality may also play a part. Reduced mortality in recent years may have increased the male excess.

Severe intellectual impairment has often been found to be evenly distributed across socio-economic groups, a surprising finding given the social class differential of most measures of morbidity and mortality, and the social factors involved in the causes of severe intellectual impairment. Some more recent studies show the expected social class gradient and it is likely that a differential in inceptions, masked by a similar differential in early mortality, is revealed as mortality falls.
Mild intellectual impairment (IQ 50–69)
Using tests validated for specific populations, IQs approximate to a ‘normal’ statistical distribution. Test means and standard deviations, must be known from recent studies, because test means change over time with developments in education and other cultural changes. That is, the test mean for a particular population may not be 100, although tests were originally designed in this way. The prevalence of mild intellectual impairment (IQ 50–69) reflects almost entirely the statistical distribution. For a test of mean 100, standard deviation of 15, 2.27 per cent of the population will fall below IQ 70 (2 standard deviations below the mean), plus a small effect from specific pathologies influencing mostly the bottom of the range. The few populations providing data confirm this with figures of 25 to 30 per 1000 (Richardson and Koller 1985).
The proportion of children in this group (IQ 50–69) identified in schools as ‘learning disabled’ depends upon the particular features of a school system and its arrangements for ‘special needs’. Children with higher IQs but a range of other problems may also be included, so the group as a whole may not relate closely to the IQ category. The relatively few adults with mild intellectual impairment identified as mentally retarded are those with the lowest IQs or those with serious additional motor, sensory, communication, or behavioural problems.
Although this group is never identified as a whole in services, it is important for research, especially studies of the effects on intelligence of exposure to hazards of birth, trauma, or environmental factors such as lead. Outcome must be assessed in relation to population norms in complete cohorts of births.
Mild learning disability and mild dependency related to intellectual impairment
Concepts of global disability and global handicap are commonly confused, and terminology is especially problematic. In the United States ‘learning disability’ is generally applied only to groups not exhibiting intellectual impairment, but in the United Kingdom it is the current general term for those identified as having some degree of intellectual impairment, i.e. what would be called ‘mental retardation’ in the United States. Other countries use yet other English terms for generalized learning disability and generalized dependency in the taxonomy described above. No doubt other languages experience similar problems.
In the context of the ICIDH, generalized learning disability is the appropriate term for the global disability experienced by people with intellectual impairment. Available statistics mostly record school or adult service groups characterized by certain levels of dependency, i.e. conceptually implying a generalized handicap definition. They do not commonly apply standardized measures and are therefore difficult to compare and interpret.
Such statistics expressed as population frequencies are usually called ‘administrative prevalence’ because they count those known to particular community service systems or derive from population registers related to service provision. They have been considered non-scientific, but if they are understood as frequencies of ‘generalized dependency’ (related to intellectual impairment), the characteristics and determinants of which in different societies can be examined and compared, they offer great potential for valuable research.
As noted above, terms purporting to describe severe learning disability and severe dependency (handicap) are frequently defined by measures of IQ. If a criterion of IQ < 50 is imposed upon either generalized learning disability or generalized dependency, the group defined is identical to, and should be identified as, severe intellectual impairment. This does not apply at lesser levels of intellectual impairment, although the same confusion prevails. The following discussion focuses, therefore, on the mild category, but the arguments apply also where mental retardation or an equivalent term is used without differentiation of degree. Because ‘mild generalized dependency’ has, as yet, no currency, the conventional term ‘mild mental retardation’ will be used, and readers must translate it into their own term of common use!
Mild mental retardation as identified in any community is never the same as mild intellectual impairment as defined above (all those with IQ 50–69 in the total population). Many individuals in that IQ range cope satisfactorily with society’s demands and are never considered mentally retarded. Indeed, it might be said that the group exists mainly because of the demands of universal education; in communities without this, very few will be identified as ‘different’. In other situations some people are considered mentally retarded in spite of an IQ above 69. Following up the British 1958 birth cohort, 10 per cent of all children were identified by teachers as needing special or remedial education, many with IQs above 69 (Fogelman and Wedge 1981).
The key to understanding this is to recognize that low intelligence, whether measured or merely clinically estimated, is only one of many criteria by which people are identified as different and dependent, and are therefore called mentally retarded (or equivalent term). Other social factors determine who is selected as mentally retarded to receive special services, and these often outweigh intelligence so that people with IQs over 69 may be included. It is important to recognize that mild mental retardation is always socially determined. The vast variation in reported prevalence for adults in the past 40 years (e.g. 2.97 per 1000 in Wessex, United Kingdom, and 77.91 per 1000 in Rose County, United States, both published in 1968), illustrates the variety of inclusion criteria. There is no standard definition, and there are no representative statistics for the prevalence of mild mental retardation.
Local ‘registers’ are valid measures if they fully record those for whom the label is perceived as appropriate in each community. We can only speak of ‘underestimated’ or ‘hidden’ mental retardation in respect of those who fulfil local criteria, but who have not been identified. In practice this is seldom known because many criteria are not overtly understood, are ambiguous, or are variable in application.
However, fruitful research is possible if the following crucial questions are recognized. Why are more people selected as mildly mentally retarded in one community than another? What are the different criteria, acknowledged and hidden, by which particular societies select people to be labelled mentally retarded? What are the consequences, good and bad, of selection or non-selection for individuals with mild intellectual impairment? Studies identifying the criteria for selection which operate in different communities and which determine prevalence, would help us to understand the social context of service provision, the variants and determinants of stigma and discrimination, the concomitants of labelling, and the advantages and disadvantages to vulnerable people of being excluded from the group. They would also guide community care planning.
Factors commonly influencing selection as mentally retarded are summarized in Table 5. Although perceived low intelligence and certain medical diagnoses may be overtly recognized, legislation, professional attitudes, and service structures and traditions may have much greater influence. For example, ‘mild mental handicap’ almost disappeared in the United Kingdom when social workers, who generally did not label people in that way, took the lead role from doctors who did. This can be justified in terms of normalization, but it may also mask unmet needs. It also renders epidemiological study very difficult.

Table 5 Factors affecting selection as mentally retarded

People with mild intellectual impairment are more likely to be selected if they also have communication problems, multiple physical disabilities, mental illness, or challenging behaviour, and if they suffer unemployment, low socio-economic status, poor home environment, or inadequate parental care. In many countries, current services will have inherited some people, especially in or discharged from long-stay institutions, who have higher levels of intelligence and who would not now be labelled mentally retarded.
It is these selection criteria, recognized or unrecognized, which lead to the commonly observed characteristics of the mildly mentally retarded group. There are few cases with a precise aetiological diagnosis, but many with neurological impairments, major and minor, and many with motor and sensory disabilities. Epilepsy, cerebral palsy, autism, mental illness, and challenging behaviour, especially that which attracts the attention of the police and law courts, are more common than in the general population.
A very strong bias towards lower socio-economic groups has long been known, giving rise historically to terms such as ‘sub-cultural retardation’. The interaction of low intelligence with poor stimulation and education, poor social environment, emotional deprivation or abuse, or lack of an effective, supportive family, will render a child or young adult very susceptible to identification as ‘a problem’, and to be labelled as mentally retarded. In a supportive family, a child of similar intelligence is likely to escape the attention of formal agencies outside school. Racial bias has also been demonstrated in some societies.
Sex distributions have always favoured males. X-linked disorders have some effect, and girls tend to be more mature than boys, age for age, but the excess is probably mostly the result of social selection of ‘unacceptable’ male-type behaviour in boys and young men of low intelligence. Dull girls tend towards more passive behaviour at school, and as young women tend to show less aggressive and criminal behaviour. They are often more protected at home, where they may be given satisfactory domestic occupation whereas men are perceived as unemployed. Many individuals have the basic capacity to be independent and to enjoy an essentially normal lifestyle. Successful rehabilitation, or simply maturing with increasing age, may move people out of the category as they get older.
Selection criteria change over time in all societies. In developed countries, changes in the philosophy of care, diversification of professional involvement, and diminishing reliance on legal control or formal classification have led to fewer people being labelled as mentally retarded. However, these factors and processes operate differently in different communities, and without understanding them we can offer no interpretation or explanation of differences n prevalence of mild mental retardation, nor can we plan appropriate services.
Central nervous system impairment: aetiological diagnosis and prevention of intellectual impairment
Known pathological processes and syndromes which contribute to intellectual impairment through damage or disorder of the central nervous system are very many and mostly rare. In mild intellectual impairment, few cases have a single clear biomedical cause; aetiology lies in the interaction of polygenic inheritance, neurological impairments, and social factors. In severe intellectual impairment, all cases have demonstrable pathology, although only 80 per cent or so will be diagnosed in life even with good paediatric assessment.
However, a diagnosis does not imply a fully understood cause, which is usually a complex of causal processes and causal factors related to particular outcomes. The rubella virus can be said to cause rubella syndrome, but what determines maternal infection, fetal infection, and fetal response to infection? In Down’s syndrome we know much about the processes of causation, but little of the ultimate causal agents. The same processes and factors which may result in severe or mild intellectual impairment, may also diminish intelligence at higher levels, or result in a neurological impairment with no effect on intelligence. Aetiological studies must relate to particular causal factors and particular pathological outcomes; multiple exposure variables and multiple outcome variables almost always preclude clear conclusions, as illustrated by the many studies of perinatal factors.
Types and frequencies of organic syndromes
Table 6 orders organic syndromes by the period of development of the principle aetiological factors, with a brief description, estimates of frequency, relationship to intellectual impairment, and potential for prevention. A few syndromes are major contributors: iodine deficiency in some communities; fetal alcohol syndrome in some; Down’s syndrome and fragile X in most. Other syndromes are rare, and small numbers make causal studies and evaluation of preventive programmes difficult, requiring enormous populations. Thus most frequency data are uncertain, approximate, and provisional. The three groups of organic causes are summarized below.

Table 6 Aetiology of organic syndromes related to intellectual impairment (frequencies are approximate and sometimes insecure)

Pure primary disorders are present from conception, an autosome or sex chromosome aberration in one gamete producing an abnormal chromosome constitution. Trisomy 21 is the archetype, but trisomies 13, 17/18, and others also occur. Sex chromosome disorders are seldom associated with significant intellectual impairment. X-linked disorders gained prominence with the elucidation of fragile X syndrome.
Primary disorders with secondary neurological damage do not affect the general constitution, but a genetically determined specific defect affects development, with or without environmental provocation. Phenylketonuria is the most common (0.05 to 0.2 per 1000 births) of many inborn errors of metabolism, in which absence of an enzyme disrupts metabolism of proteins, carbohydrates, lipids, or mucopolysaccharides. Brain damage and severe intellectual impairment arise if the hazardous dietary component is not excluded from infancy. Sporadic congenital hypothyroidism leads to cretinism if not identified and treated with thyroxine (T4) from infancy.
Pure secondary disorders arise from an environmental insult to a normal zygote any time after conception. Prenatal causes include neural tube defects, iodine deficiency disease, rhesus incompatibility, and the effects of communicable diseases and toxic agents such as alcohol, drugs, and radiation. Perinatal processes are complex; the main factors are hypoxia, hypoglycaemia, cerebral thrombosis and haemorrhage, and gross trauma. Factors increasing vulnerability in the baby include very large baby, very small baby, immaturity, and pre-existing abnormality. Other factors lie in the mother. Postnatal causes include encephalitis and encephalopathy from communicable disease, trauma, and metabolic disasters in infants.
Cause, frequency, and prevention
Causes in individuals must be considered differently from causes in populations. For an individual case, one asks what factors contributed to the damage or disorder observed; for a population, one asks what factors contribute to the prevalence in that population. This is particularly important when planning preventive interventions. However ‘biological’ a known cause of neurological impairment may be, there will be social and environmental factors which increase or decrease its frequency in populations. We may not be able directly to intervene in the causal process, but we may be able to influence factors which reduce prevalence. For example, we do not know the underlying cause of non-disjunction in Down’s syndrome and cannot intervene, but the prevalence of Down’s syndrome is influenced by age of marriage, family size, attitudes to contraception, economic circumstances, education, religion, and the law, which act variably on inception rates and mortality. Even if only maternal age is reduced, birth prevalence will decrease.
The prevalence of recessive genetic disorders is determined by consanguinity, religion, and isolation. Fetal alcohol syndrome depends upon cultural traditions, attitudes to alcohol consumption in pregnancy, and government tax policies. Perinatal damage reflects early nutrition of girls, traditions of child birth, and distribution of services.
Similarly accidents, infectious diseases, neural tube defects, iodine deficiency disorders, and others are determined by environments, traditions, attitudes, government policies, and other factors. For most people with intellectual impairment, a medical diagnosis offers little scope for treatment or any advantage for habilitation, but for communities, diagnosis may offer many practical options for reducing incidence and prevalence mainly by manipulating social factors.
Prevention of intellectual impairment may be one of many justifications for social action, for example to reduce road, industrial, and home accidents, all common in both developed and developing countries. General economic and social development is important, and especially the situation of women, their economic status and roles within the family. A co-ordinated efficient service infrastructure is probably more important than high technology, but an appreciation of scientific issues in policy-makers is helpful. Both the prevention of intellectual impairment and the development of rehabilitation services are dependent on social and political stability.
Specific aetiological groups: causal processes, treatment, and prevention of the most important syndromes
Down’s syndrome
Down’s syndrome is a universally common cause of intellectual impairment, with IQs falling mostly within the range 20 to 55. About 94 per cent are due to chromosome non-disjunction leading to trisomy 21, of which birth frequency increases with maternal age: about 0.5 per 1000 at age 20, 1 per 1000 at 30, 2.5 per 1000 at 35, 10 per 1000 at 40, 40 per 1000 at 45, and more than 150 per 1000 at 50, but with some racial variation (Huether et al. 1998). The most effective strategy for prevention is to reduce births to older women. This has happened in many countries by the widespread use of the contraceptive pill.
Fetal diagnosis by amniocentesis, followed by abortion, is available in most developed countries, usually targeted at older women. Prescreening with serum markers (four currently available) and ultrasound is increasingly common, but, if offered to all women, increases the total number of amniocentesis procedures with their attendant risks (Wald et al. 1998). Ethical issues are not easily resolved. Generally, no more than half of all Down’s fetuses are detected, and women are offered only a calculated risk that they are carrying a Down’s baby. Are they willing to undergo abortion if the tests reveal a high risk? Can they understand the basis on which the risk is calculated? At what level of risk should they be asked to decide on termination?
Until recently it was thought that 75 to 85 per cent of Down’s syndrome was of maternal origin, but recent DNA polymorphic analysis suggests that it may be 90 to 95 per cent. Most non-disjunctions occur at the first meiotic division, during the mother’s fetal life, remote from fertilization, and not susceptible to intervention. Increased incidence with maternal age may be related to ageing gametes within the ovary. There is some evidence that radiation could be a cause of non-disjunction at the second meiotic division. Those of paternal origin also arise just prior to fertilization; exposure to industrial toxins may be implicated, but intervention is not yet practicable. Of non-disjunctions 1 to 3 per cent show mosaicism, with a mixture of normal and abnormal cells and a variable picture of features, impairments, and disabilities. This probably explains the rare reports of a Down’s person of normal intelligence and achievements. Three to five per cent of Down’s syndrome is familial, due to gene translocation. Chromosome diagnosis is important to detect those for genetic counselling.
The typical features of Down’s syndrome are recognizable in all races usually at birth, and include epicanthic folds over the eyes, a flattened upper nose, a narrowed cranium, and a small mouth, often with a protruding tongue. Hands and fingers are short, with only one prominent skin crease. Between 25 and 40 per cent have a serious heart defect, and many have increased vulnerability to infectious diseases, both major causes of early death. First-year mortality in developed countries has decreased from 50 to about 10 per cent, and many individuals now live at least into late middle age (Table 3) (McGrother and Marshall 1990; Hayes et al. 1997). In developing countries where early mortality is high, few Down’s children survive. Hypothyroidism, leukaemia, and diabetes are relatively common in adults, and Alzheimer’s dementia is common in those who reach middle age.
Fragile X syndrome
The prevalence at birth of this sex chromosome disorder is still subject to improving diagnostic techniques; recent studies suggest much lower figures than previously estimated, probably 0.2 to 0.3 per 1000 (Morton et al. 1997). A majority of affected boys have IQs below 70, but a minority of boys, and virtually no girls have IQs below 50. About 70 per cent of girls who carry and can transmit the genetic abnormality are otherwise unaffected; most of the others are mildly intellectually impaired. Clinical diagnosis is unreliable, but men with severe intellectual impairment show a variety of physical abnormalities including, commonly, large testes and long ears. They may show autistic behaviours. Screening of groups of mentally retarded men permits genetic counselling of family members about risks for future children. Population screening to detect carriers is barely feasible and not yet evaluated (Murray et al. 1997). Ethical and resourcing issues need addressing before the pressures of technical advance pre-empt them.
Inheritable metabolic disorders
Congenital hypothyroidism occurs sporadically in all communities (0.1–2.0 per 1000) unrelated to iodine deficiency, usually as a mutation.
Thyroid failure leads to cretinism, with severe intellectual impairment, if not detected and treated with T4 before 3 months of age. In most cases early treatment will largely preserve intelligence, but irreversible damage may occur in some before birth. In the variety called thyroid agenesis, very low T4 levels at diagnosis, and treatment as late as 3 weeks of age, may result in some loss of intelligence, although not enough to be identified as mentally retarded. Treatment is, therefore, extremely urgent. Comprehensive screening programmes are now widely adopted using a simple blood spot test and laboratory analysis immediately after birth.
Logistically, these have often been mounted on longer established blood screening programmes for phenylketonuria (0.05–0.2 per 1000 births) and several other very rare enzyme disorders. In these, a defective gene leads to an enzyme deficiency and accumulation of toxic byproducts from normal constituents of diet, resulting in progressive brain damage and severe intellectual impairment. A special diet can prevent serious damage and allow children to develop with their intelligence relatively unaffected. The diet is not easy to take and must be continued at least into late childhood. Loss of intelligence is related both to the age of starting the diet and the control of blood phenylalanine. It is therefore of great importance to identify children very soon after birth, to start the diet immediately, and to monitor and maintain it closely throughout childhood. Optimal screening and treatment leads to no measurable loss of intelligence. The diet must also be reinstated during pregnancy to protect the fetus (Friedman et al. 1996). Such programmes are demonstrably effective and cost beneficial in developed countries, and lessons can be learned from the longest established programmes, such as that of 25 years in the United Kingdom (Seymour et al. 1997; Streetly and Corbett 1998).
Recessive genetic abnormalities in general
Although each specific syndrome is very rare, collectively they are important. Physical anomalies often predominate and may be susceptible to surgical correction. There may be any degree of intellectual impairment or none. They are thought to be mostly mutations, but, in communities with a high degree of consanguinity, a substantial proportion may be familial. Consanguinity may be a cultural tradition, or a product of geographical isolation (e.g. remote mountain or island communities), social isolation (e.g. elite groups or those marginalized by poverty), or cultural isolation (e.g. immigrant groups). Prevention requires education, but without relief of isolation this may have little effect. Because of the frequency of mutations, the avoidance of mutagenic factors in the environment might be important, including protection of industrial workers and communities from chemical or radiation exposure.
Neural tube defects
Neural tube defects are sometimes associated with intellectual impairment, especially where there is hydrocephalus. Epidemiologically, they show a clear geographical variation in incidence in many countries, and dietary folate deficiency is established as an important cause, possibly reflecting a metabolic fault. Any supplement is required before conception, as the damage often occurs before pregnancy is recognized. Since the risk is greatly increased after one affected baby, folate supplements (4000 µg/day) should always be given to prevent second neural tube defects. Large-scale studies have shown that incidence can be reduced by changes in diet or specific folate supplementation (Locksmith and Duff 1998), and all women who are at risk of pregnancy should ensure adequate folate intake of 400 µg/day. Supplementation is cheap and tablets easy to take, but information and encouragement alone have proved inadequate (Rosano et al. 1999). Some countries are adding folate to common foods, such as cereals in the United States, but the adequacy of the amount is disputed (Daly et al. 1997). About a third of neural tube defects may be resistant to folates. Environmental lead may also be a causal factor (Watkins 1998).
Fetal alcohol syndrome
Frequencies are variable, with high incidence in some communities, for example native Americans on ‘reserves’ with seriously prejudiced cultures, lack of employment, and high consumption of alcohol. In some countries high risk will be limited to certain subgroups and deviant individuals; this may explain the apparently much higher rates in the United States than Europe (Abel 1995). It is not yet certain what dose of alcohol represents a calculable risk of the full syndrome including severe intellectual impairment. There is evidence that damage less than the full syndrome may also arise from alcohol consumption (Bradley et al. 1998). The best advice is still to avoid alcohol completely during pregnancy. Further prevention means tackling alcohol consumption in general (Forrest and Florey 1991; Alberman 1992).
Iodine deficiency disorders
Worldwide, iodine deficiency is the most common cause of mental retardation. Iodine is essential for the production of T4, without which metabolism is slowed, and all growth and development inhibited. Adults obtain the 150 µg/day they need mostly from water, but in many upland areas it has been leached from the soil and is insufficient. Variation in individual response to environmental iodine is not understood; other minerals, goitrogenic toxins in food, and genetic factors may each play a part.
In 1985 the WHO estimated 6 million people with cretinism in the Southeast Asian region alone, not including China (WHO 1985). In China, more than 10 per cent of some rural populations can show frank ‘cretinism’ with many more exhibiting goitres and varying degrees of growth retardation, although preventive programmes are now changing this (Wang et al. 1997). The estimated populations at risk are over 1500 million worldwide, 29 per cent of the world population, with huge numbers in China, India, and other Himalayan, Asian, South American, and African countries. It is estimated that 50 to 100 million people are at risk in Europe. At least 110 countries have an iodine deficiency disease problem (WHO 1993, 1994).
Endemic congenital hypothyroidism (cretinism) combines intellectual impairment, stunting, deaf–mutism, and neuromuscular disorders in varying degrees. Treatment usually produces no clinical or intellectual improvement, although rehabilitation can improve quality of life. Whole communities suffer poor levels of economic performance, education, and social life, and need rehabilitation.
Prevention is technically simple, effective, and cheap. Unfortunately, it is not always logistically or culturally easy to introduce and maintain. To increase iodine (preferably iodates for stability) in the diet of a whole community, the most common vehicle is salt, used by everyone throughout the year. With a limited number of sources, costs in 1988 could be around US$20 to 40 per 1000 people per year. However, poor quality control, distribution delays, lack of acceptance, and use of alternative sources of salt threaten the effectiveness of many programmes (Dunn 1996).
Injections of iodized oil are logistically more demanding and expensive, but more certain, and especially suitable for remote communities and fertile or pregnant women in the first two trimesters. One injection can be effective for several years, there are few side-effects, and no cold chain is required. Experience is now being gained with oral iodized oil, which enters body fat stores from which iodine is slowly released, and appears successful. Supplementation of drinking water, irregation water, bread, and common sauces have also been used. In some communities oral potassium iodide tablets may be appropriate.
The prevention of iodine deficiency needs political and professional will for success. The world programme to eradicate it by 2000 has done well but is not complete; every country with iodine deficiency disease needs a strong campaign (van der Haar 1997). It will be a generation before cretinism disappears from a community, but tangible benefits will be perceived in only a few years, and in the long run, a successful programme will pay off abundantly (Hetzel 1989).
General nutrition
Although many have asserted that gross maternal or early infant undernutrition can damage the central nervous system and cause irreversible intellectual deterioration, this is not supported by the evidence. The known pattern of early brain development offers ready explanation of both fetal and infant protection. The second period of brain growth, from 20 months of fetal life to 2 years of postnatal life, permits repair of any spongioblast damage up to that age (Stein et al. 1975; Dobbing 1984; Fryers 1990).
However, generalized malnutrition contributes to increased fetal and infant vulnerability to micro-organisms and other hazards. It may also contribute to poor learning function and delayed development (Grantham-McGregor et al. 1991). Children who experience severe prolonged malnutrition and grossly deviant social rearing, may present as mentally retarded yet have the capacity to recover normal mental and social function, and it is very important to identify such children and treat them with food, stimulation, love, and care. This may not be easy; children may be assumed to be permanently intellectually impaired, especially if in institutions without informed professional supervision. Most permanently impaired children can also develop more than professionals generally expect, with appropriate help. The important lesson is that all retarded children should be fully assessed and given good physical, mental, emotional, and educational care from the earliest age possible, to maximize their potential and minimize their dependence. Signs of unexpected progress should prompt active reinvestigation.
Perinatal factors
During the perinatal period, a baby faces several hazards—anoxia, hypoglycaemia, thrombosis, and trauma. We rarely know its fitness for birth, although very low weight for age or gross prematurity are strong indicators, or the mother’s preparedness for delivery, which varies with age, parity, health, and nutritional state. The result may be neurological impairment, such as cerebral palsy and epilepsy, as well as intellectual impairment. But many cases, particularly of cerebral palsy, are now thought to arise earlier in pregnancy, and similar syndromes may also arise in infancy from meningitis and encephalitis. In populations, the relationship between healthy survival, damaged survival, and death is dynamic. Where many babies are damaged, many also die. If conditions improve, fewer babies may be damaged but more damaged infants may survive; prevalence at subsequent ages will not necessarily be lower (Johnson et al. 1993).
The scientific evidence is difficult to evaluate (Fryers 1990; Escobar et al. 1991; Dammann and Leviton 1997), but perinatal factors, not least very low birth weight, are without doubt important causes of intellectual impairment, cerebral palsy, epilepsy, and mortality, and all may be reduced in frequency by effective antenatal and obstetric care. This may mean improving nutritional state of mothers and young girls, increasing access to early antenatal care, training traditional birth attendants, providing professional midwives, building up the primary health care system, and ensuring skilled hospital obstetrics where required.
Communicable disease
Damage to the fetal or infant brain may arise occasionally in many infections (Table 7), although establishing a causal relationship is often difficult.

Table 7 Principle infectious agents causing intellectual impairment

Endemic and epidemic variation precludes even guideline incidence rates, but if a particular disease is very prevalent, especially in a poor community, it is reasonable to assume that there are risks, especially of meningitis and encephalitis. Some regions, particularly in Africa, have especially high rates of meningitis.
Cytomegalovirus is common and may cause more damage than generally realized, including intellectual impairment and deafness. We have as yet no specific means of prevention. Rubella syndrome can be prevented by immunization, and congenital syphilis by early diagnosis and treatment; intellectual impairment is rare. The vaccine for Haemophilus influenzae type B brings hope for further immunization against bacterial meningitis and encephalitis. Chemotherapy is still important. Encephalitis in infancy carries a high risk of intellectual impairment, cerebral palsy, epilepsy, and death, and careful clinical handling may both determine individual outcome and prevent secondary cases.
Toxoplasma gondii is a common protozoal infection spread from animal faeces or poorly cooked meat. Fetal infection does occur, but signs are variable and may be late. Any degree of intellectual impairment can result, but risks are difficult to compute (MDWG 1992). Incidence varies, and there is no consensus on screening policy; universally offered in France, it is still debated in the United Kingdom and United States. Otherwise, prevention relies on good hygiene, but antibiotics during pregnancy may help.
Malaria increases vulnerability to other infections, and may sometimes damage the fetal or infant brain directly. Measles can occasionally cause late-onset damage which may include intellectual impairment with subacute sclerosing panencephalitis. High levels of population immunization are effective in prevention (Miller et al. 1992; Aaby et al. 1993).
Prevention of intellectual impairment due to communicable disease largely depends upon general public health measures—provision of safe water, waste water treatment, control of vectors, general hygiene, case isolation, early diagnosis and effective treatment, and improved resistance through good nutrition. Specific prevention by immunization is important. It provides individual protection but, more importantly, herd immunity if immunization rates of at least 90 per cent and preferably 95 per cent can be achieved to restrict spread. The measles–mumps–rubella (MMR) vaccine is very successful; pertussis and tuberculosis are also important. Some vaccines, particularly pertussis and MMR have been alleged to cause brain damage as occasional side-effects. Evidence for this tends to be circumstantial, and extensive scientific reviews have failed to confirm these risks, but proof of no risk by epidemiological assessment is very difficult. The public health consensus is that if there are risks, they are extremely small and are far outweighed by the risks of damage from the wild virus in the absence of immunization.
New vaccines are constantly becoming available, but the effectiveness of immunization programmes is determined by the quality of primary health care and public health organization. For more detail, regularly updated handbooks of communicable disease (Benenson 1990) should be consulted.
Commonly associated syndromes of impairments and/or disabilities
Certain complex syndromes, although separate entities, are frequently associated with significant degrees of intellectual impairment and are always prominent in groups of people defined as mentally retarded. Congenital abnormalities tend to occur together in populations and sometimes in individuals. Epilepsy and cerebral palsy commonly arise from the same aetiological process as intellectual impairment before, during, or after birth. Proportions vary with the way mentally retarded groups are defined, and with the spectrum of aetiologies in different communities, but epilepsy is reported in 20 to 50 per cent of people with severe intellectual impairment (IQ < 50) (Hannah and Brodie 1998). Many cases can be controlled with medication. It is a specific component of tuberose sclerosis. Cerebral palsy is reported in 15 to 40 per cent of those with severe intellectual impairment, depending on the thoroughness of neurological examination. There are few data from developing countries, but both cerebral palsy and epilepsy are likely to be common. Children with cerebral palsy and epilepsy need medical assessment, treatment, and supervision to minimize the physical disabilities so that optimal development of intellectual abilities, at whatever level, can be promoted.
Autistic spectrum disorders are often associated with intellectual impairment but, given the profound and complex communication problems characteristic of autism, this should not be assumed without specific evidence. Strictly defined, autism is probably only 2 to 4 per 10 000 of the child population in developed countries, 50 to 75 per cent of whom may be considered to be mentally retarded. Using broader case definitions, autistic spectrum disorders are more prevalent; some, like Asberger’s syndrome, are less likely to be associated with intellectual impairment. Another syndrome has been described, the ‘triad of social and language impairments’ (Wing 1996). There is evidence of gender and race differences in patterns of disability and behaviour. Variable neurological response to similar environmental insults is possible.
Many studies have reported higher prevalence rates of psychiatric illness in those designated mentally retarded, but there are serious problems of interpretation when neither term is clearly or consistently defined. Non-psychotic disorder is probably common, related to brain disorder, experience of abnormal social and interpersonal environments, or other impairments and disabilities. People identified as mentally retarded tend to be subject to closer surveillance than the rest of the population, not least by psychiatrists, who thus have unusual opportunities to make diagnoses. However, psychiatric examination is often extremely difficult in people with severe intellectual impairment, and many psychiatric disorders described in the literature are minor or ambiguous, and overlap with categories of challenging behaviour (Smith et al. 1995).
Anxiety and depression are susceptible to treatment and are almost certainly recognized less frequently than they should be. Schizophrenia has often been said to be found in ‘3 per cent of the mentally retarded’, but comparing lifetime prevalence has proved almost impossible since definitions of both groups are so variable, and the only good, full community study produced figures of 1 per cent like the rest of the population (Turner 1989).
Behaviour disorders are not necessarily comparable phenomena in people with and without serious central nervous system impairment. But there is no doubt about the importance of the relatively few intellectually impaired people who exhibit serious challenging behaviour, because of their impact on families, communities, and services. This is especially true for people with relatively high IQs who are also fully mobile (Mansell 1993).
Problems arise not only on account of ambiguous definitions, but also because the presence of several impairments and/or disabilities makes examination, diagnosis, and assessment extremely difficult. It should also be remembered that any concurrent disability, disease, or deviant behaviour may render an individual more likely to be identified as ‘mentally retarded’ by health workers, social workers, police, or the courts, so that such problems are inevitably over-represented.
Specific disabilities
Identification and assessment of mentally retarded people
As well as learning disabilities and the common disability syndromes, many people with intellectual impairment suffer motor and sensory disabilities which have a serious impact on their daily lives. Although many clinical assessments are available, scientific measurement and taxonomic development of disabilities have received relatively little attention. There are, indeed, formidable epidemiological problems in standardizing instruments and discriminating degrees of disability. Many instruments provide individual profiles of abilities and disabilities to inform individual programme planning, but do not permit comparative study. An exception, Wing’s Handicaps, Behaviour and Skills Schedule has been used for comparative research in several countries (Wing 1980; Ort and Cooper 1984) and generated a short Disability Assessment Schedule suitable for studies in service settings (Holmes et al. 1982).
Those studies showed that mobility and degree of social interaction are better indicators of demanding behaviour and need for care, than IQ. Mobile children with poor social interaction are particularly difficult to manage in any setting, and those with higher IQs may be the most difficult of all. Prevalence rates in European-type populations are likely to be between 0.25 and 0.5 per 1000. About three-quarters of these are severely intellectually impaired; a few would have IQs above 70.
It has already been noted that epilepsy is found in 20 to 50 per cent, and cerebral palsy in 15 to 40 per cent of people with severe intellectual impairment. Many motor and sensory disabilities are commonly associated with intellectual impairment, increasing in frequency with lower IQ. Serious visual problems are found in 10 to 30 per cent of people with severe intellectual impairment (IQ < 50), hearing problems in up to 5 per cent, and serious problems with speech in 60 to 85 per cent. The communication problems which result from these often outweigh all other social consequences; they make assessment difficult to perform and services difficult to provide.
These features of the population must be taken fully into account when planning care, especially the provision of personnel, but these generalities do little to guide care for individuals. Thorough, individual assessment of all impairments, disabilities, and abilities is essential if individual children and adults are to be given help specific to their needs. Standard forms should be used wherever possible in order to promote clear thinking and encourage well-planned programmes of education, habilitation, treatment, and care, tailored to individual need and susceptible to rigorous monitoring. Clarifying variations in client behaviour and performance in different environments will also encourage evaluation of different interventions.
It should be noted that assessments of function in restrictive, institutional settings, or of people whose experience has been largely in such settings, may exaggerate problems and underestimate individual potential. It is a common experience in practice to find severely intellectually impaired people doing far better in community settings than expected, in terms of self-care, domestic skills, social competence, and degree of independence.
Consideration of the range of disabilities experienced by intellectually impaired people also implies that ‘good medical care’ means carefully planned, positive habilitation or rehabilitation, with clear objectives and professional techniques of intervention applied to all areas of disability. Professional contributions are required, where possible, from physiotherapy, speech therapy, occupational therapy, psychology, family doctor, paediatrician, and psychiatrist, as well as the skills of teachers, nurses, social workers, and carers. Medical and surgical treatment should not be neglected where specific functions can be improved. Behaviour modification and other psychology programmes should be professionally designed; they can crucially improve behaviour and permit learning and development otherwise inhibited. However, professional staff and family carers should also recognize the abilities and resources which intellectually impaired people bring to their own needs, and build upon them to extend the scope of their experience and enhance their lives.
In well-resourced services, many people are involved in the ‘team’, and there is a danger of unco-ordinated, poorly focused, and inefficient services. The family may be neglected as the principle source of care for many clients, but also in respect of their own needs. Even the client’s needs and preferences may be lost sight of in the complexities of team work. The concept of the ‘key worker’ has proved invaluable in addressing these problems, especially in community-based, multidisciplinary, and multiagency services. But teams are necessary—sharing knowledge, skills, and experience is essential when disabilities are so wide ranging and so disadvantageous.
In developed countries, severely mentally retarded children will usually be identified in infancy through routine developmental examinations. In countries with few services for mentally retarded children, children may not be identified because there seems no advantage in doing so. However, mothers have usually observed developmental delay or other problems, and their anxieties need to be seriously addressed. Even where no schooling is available, parents can be supported and advised on early stimulation, play, and teaching that they can do at home. School systems frequently identify children with milder degrees of intellectual impairment, but in many developing countries obviously retarded children never start school.
In any culture, a sense of shame can drive parents to hide an obviously disabled child, sometimes with the collusion of officials and professionals. Conversely, special services for retarded children in many countries were started by parents themselves long before wider recognition of the need. This process continues in developing countries, but not all families are involved. Screening programmes or surveys to identify unknown cases should only be undertaken when services can be offered to those identified. When children are suspected of delayed development, parents need to be fully informed and involved throughout the processes of identification, diagnosis, and assessment. Once identified, parents need careful counselling to help them to come to terms with their new reality; this may be done by professionals, but carefully chosen parents who have been through a similar experience themselves can be very effective.
Assessment poses different problems in developed and developing countries. In the former, many professions claim specialist expertise. Each has a professional orthodoxy and each has a range of conventional assessment techniques and instruments. Co-ordination by a key worker is essential if children and families are not to get lost in the maze, and expensive resources are not to be wasted. In developing countries, the few specialists are usually confined to urban centres. For most children, assessment must come from primary health workers and local school teachers who need training in the recognition of mental retardation, in developmental and functional assessment, and in the potential for habilitation within the family and community.
Assessment should not be used to exclude people from services, but as a starting point for action. Assessments are always age related and compare performance with norms from a reference population (which is not always appropriate) in order to identify immediate needs, and to predict and monitor progress. It is important to assess abilities as well as disabilities, strengths as well as weaknesses, as these represent vital resources for the future. Where possible, assessment should include both formal testing and less structured observation in familiar environments. There are many scales, tests, and checklists; some are suitable for population screening, others for individual assessment. These are described in Mittler’s (1992) wide-ranging summary and annotation, and Hogg and Raynes’ (1987) critical review. Few tests are validated for developing country populations, and must therefore be used with caution.
Sensory assessment is important, as poor sight and especially deafness can prejudice the assessment of development and intelligence. Hearing tests need special equipment and training. Motor disabilities, epilepsy, cerebral palsy, and psychiatric illness should be fully assessed, for which standard protocols are increasingly used, as doctors are generally poorly trained in functional assessment.
Intelligence tests giving IQs have been in and out of fashion. Their potential and problems are usefully discussed in Graham and Lansdown (1992). They are not independent of education, experience, and culture, and should always be repeated, should include subscores, and be accompanied by an interview report. Shortened tests are available for use by less trained workers. Development tests use age-related ‘milestones’ for young children (e.g. Gesell, Griffiths, or Bayley tests). Few formal tests are standardized for developing country populations, and are thus of doubtful validity; behaviour checklists may be more useful to guide specific interventions, as in the Portage system.
Social adaptation is assessed to guide rehabilitation. The American Adaptive Behaviour Scale is relatively complex, examining personal independence and maladaptive behaviour in over 20 areas of social function such as self-direction, responsibility, and socialization. The Vineland Social Maturity Scale is simpler but more used for research. The Progress Assessment Chart (Gunzberg 1977) is used in developing countries successfully, providing a progressive visual record of mobility, self-care, practical skills, and communication. Others have been developed locally.
Social disadvantages can only be assessed in a specific cultural context; there are no standard methods. They often represent the greatest burdens and needs. Table 8 gives examples of key questions which should be addressed.

Table 8 Assessment of disadvantage—examples of questions to be asked

Dependency: community needs and organization of services
History, organization, and philosophy of care
Since the latter part of the nineteenth century, intellectual impairment has gradually emerged as a distinct and significant problem in all communities. Severe mental retardation was discriminated from serious mental illness and the extension of primary education revealed those with serious learning disability. But the determinants of behaviour were ill-understood, and identified mental retardation was clearly associated with poverty and social deprivation. Eugenicists aroused fears of degradation of the population if such people were permitted to reproduce freely. In many countries, repressive legislation promoted special institutions, usually remote, custodial in character, and housing between 50 and 5000 or more people. They were generally poorly resourced and attracted poor quality and ill-trained staff, yet they constituted almost the only service available for people with mental retardation, hiding them from public view, and reinforcing stigma, prejudice, and pessimism.
Over the past 30 years or so, communities have been struggling to overcome this tragic inheritance and to build humane, individualized, and professional community-based services promoting self-esteem, dignity, independence, integration in ordinary society, and the enjoyment of normal lifestyles. Most of the dilemmas of current services reflect this radical change, which can be seen in almost all countries, although at different stages of development and with some cultural variation. The motivation for change came from increasing activity by parents and parents’ associations, an increasing professional awareness of the human rights of people with intellectual impairment, and political responses to public exposure of scandalous practices and conditions in institutions.
Parents have frequently led the way towards modern services. Most parents never wanted their children removed into institutional care, but acceded to it in the absence of alternatives. They came together in local associations to provide mutual support, to demand better community services from the statutory authorities, and themselves to provide play groups, schools, day centres, workshops, respite care, leisure facilities, holidays, and other services. In many countries, these services were later adopted, funded in full or part, or partnered by government agencies, and many patterns of organization can be found. They particularly helped to stimulate the development of special education and adult day services, which made it so much easier for them to keep their sons and daughters at home.
It is common for intellectually impaired people to be the object of fear, ridicule, and discrimination, and for their families to experience shame and guilt. Segregated institutional services tend both to reflect and to reinforce negative images and attitudes in society, and such stigma permits devaluation and encourages maltreatment. The movement against institutions emphasized basic human rights and the necessity, not merely of improving standards of care in institutions, but of creating radically different services with objectives far beyond care: promoting satisfactory ordinary lifestyles for mentally retarded people, integrated in ordinary communities, participating in roles and activities valued in their own cultures, developing their abilities to the full, and in control of their own lives as far as possible.
These processes, and the philosophy which underpins them, are commonly called ‘normalization’, and operate according to the ‘ordinary life’ model. They require the overt presence of mentally retarded children and adults in communities: only when they are accepted as part of society’s everyday experience will stigma be seriously challenged, and their and their families’ rights to as fulfilling a life as lies within their capacity, begin to be realized.
To promote personal independence is an important principle for services, but is difficult for people with severe degrees of intellectual impairment or multiple disability. Parents must be fully involved, especially for children, but there can be conflict of interest for parents as principle caregivers, and as people with their own needs and aspirations. If such conflicts are to be sensitively resolved, professional support must be local and accessible, with no barriers of stigma, and mediated by personal workers well known to and accepted by the clients.
More recently, ‘advocacy’ has been promoted to guarantee legal and civil rights, to protect from abuse or degrading treatment, to demand information, and to assist very limited clients to make known their own desires and responses. ‘Normalization’ as a complete philosophy has its critics (Brown and Smith 1992) and it is probably best to see it as a set of valuable ideas and principles which must always be worked out pragmatically in particular situations and for particular people.
A revolution in care
Thus organizational development over the last 30 years has focused upon restricting new admissions to institutional care, relocating existing residents into ‘the community’, and building up a range of small-scale accommodation, education, work and leisure facilities, networks of social support, and access to generic services, which make ‘the community’ a practical reality.
The costs of this process are not negligible: high-quality community services may cost no more than high-quality institutional care, but most institutional care had been under-resourced and was not of high quality. Moreover, where most of the mental retardation budget was tied up in institutions, it could not be released for the development of alternatives until sections, and ultimately a whole institution could be closed. Thus extensive ‘bridging’ funds have been required. The process also required changes in location, training, and attitudes of staff, with important implications for professional recruitment, training, and organization. Nevertheless, many countries in Europe, North America, and Australasia have moved a long way towards total closure of the old institutions and their replacement with comprehensive community services, not least the United Kingdom where the national programme has been spread over 20 years. It must be emphasized, however, that closure must not precede the provision of alternatives, as has happened in some countries with dire consequences.
Professional rigidities and vested interests pose serious problems. It can be argued that medical, specifically psychiatric, domination, characteristic of institution-based services, has generally damaged the interests of intellectually impaired people and their families, despite much committed, compassionate, and imaginative work by some doctors. It is safer, healthier, and more creative for responsibility to be shared between the many professional groups who have a role in serving clients’ needs.
Increasing professionalization has characterized both social workers and nurses, who find the relative independence of domiciliary work attractive, but other professionals are also involved in well-resourced services, each reflecting the emphases, limitations, and culture of their own profession’s training and traditions, and the objectives and constraints of their own employing agencies. There is a danger of conflicting agendas, lack of co-ordination, and waste of resources. The need for multiprofessional and multiagency co-ordination, planning, and evaluation of an increasingly complex network of diverse services places mental retardation firmly in the domain of public health. The principles of human services are illustrated in Table 9.

Table 9 Principles of human services

Although there are many patterns of service organization, most have some form of local, multidisciplinary team to provide specialist advice, support, and counselling, to mediate access to services and welfare benefits, and to refer for medical and other specialist help. Some service systems do far more, controlling networks of residential and day care or managing a complex of services for a whole community. They frequently operate from a designated community centre, which may be a wide-ranging resource for mentally retarded people and their families. A key-worker system is favoured by many to combine personal access for clients with co-ordination of a team of several members.
Mentally retarded people’s human needs are primarily the same as other people’s: love and parental care, family life, a range of relationships, and opportunities for valued activities. Like others, they need access to professional help—teachers, doctors, and the like—but they may need more early stimulation, education, and training especially in self-care, practical literacy, social skills, and vocational skills. They usually need extra encouragement and support in establishing and maintaining themselves in any aspect of normal community life. A comprehensive community-based service complex will be coherent and co-ordinated within a definable community. It will encompass a home to live in, education, work and training for work, leisure, access to generic services, and provision of specialist services according to need. Each is discussed below.
Developing countries
Institutional models of care can be found in many developing countries, but almost everywhere, most retarded children and many adults, remain at home. Until recently, professionals and politicians largely ignored the needs of mentally retarded people and their families, and initiatives have come largely from parents and the organizations they have created. The International League of Societies for Mentally Handicapped Persons (now Inclusion International) helps member societies in many ways including ‘partnerships’ between those in developed and developing countries.
The International Year of the Disabled Person 1981 gave great encouragement; although situations vary greatly, the basic philosophy, principles, and elements of good practice are largely the same (Mittler and Serpell 1985). Developed countries may learn much from recent experience in developing countries, where limited funds have ensured appropriate local use of technology and the resources of families, social networks, and community volunteers, supported by specialists only when necessary.
The model has become known as community-based rehabilitation, and programmes often encompass all types of disability. Even where few specialist services exist, much can be achieved, and teaching and training materials are now available to guide those with little special experience. The model emphasizes appropriate technology and use of locally available people and resources, in the belief that effective local action which brings basic rehabilitation to many disabled people is better than waiting for sophisticated rehabilitation in urban centres serving few. Materials include the WHO manual Training in the Community for People with Disabilities (Helander et al. 1990), a collection of many simple and lucid ‘training packages’ which can be freely copied and distributed, the book Disabled Village Children (Werner 1987), and a regular newsletter CBR News from Healthlink Worldwide in London. These and many others also address rehabilitation for motor and sensory disabilities commonly associated with intellectual impairment.
An effective community-based rehabilitation programme would have community-level programmes for individual retarded people and their families run by local rehabilitation workers and supervisors. Special school facilities and day centres would offer a variety of training, sheltered work, and leisure opportunities. Specialist professionals from health centres, hospitals, colleges, and social welfare agencies, would support the programmes with guidance, staff training, and consultation, and provide access for some individuals to treatment or training in appropriate specialist centres. This model can be applied at many different levels of resourcing, but it always requires local community, professional, and political commitment.
Community-based rehabilitation projects have been set up in many countries by committed individuals, local non-governmental organizations, governments, and international agencies including the WHO and UNICEF. Evaluation is difficult, but experience is increasing understanding of the conditions for success. However, most programmes remain small scale and rural, and expanding to cover large populations has proved difficult, although there are now some examples, for example, in India. Some countries (e.g. Thailand) are grafting community-based rehabilitation onto a primary health care system. Even in cities, a community-based rehabilitation approach and overview may help to provide a rational structure for service development within which parents, professionals, and government services can co-operate.
Several training courses have been developed for community-based rehabilitation workers, but training should be as much as possible in the community where there is a community-based rehabilitation programme. It is important that disabled people and their families are involved in training their future professionals. Basic attitudes are important; selection is as important as training. Supervision and continuing training are essential, together with satisfactory career and pay structures, to ensure an appropriate status for them, their work, and their clients. Recommendations, with an outline curriculum, for training mid-level workers are available (WHO 1992b) which emphasize problem-solving approaches.
Components of community-based mental retardation services
Living in your own home
Intellectually impaired people have a right to enjoy a home of their own which promotes an ‘ordinary lifestyle’, minimizes stigma, and permits participation in the life of the community. Large residential institutions cannot provide this, and as they have diminished, we have rediscovered that almost all intellectually impaired adults can live in ordinary houses, within ordinary communities, as long as their special needs are met by support staff. Such staff may be only on call for those who are socially competent; they may visit regularly, attend during the day, or live-in, according to need. The best programmes limit the numbers living in one house to a ‘family’ size (usually a maximum of four to eight), but few large populations have yet achieved this comprehensively. Larger ‘hostels’, built in the first wave of anti-institutional enthusiasm, are now subject to similar criticisms and plans for closure.
Good programmes disperse their houses so that there is no aggregation in the community, and therefore no tendency to develop an enclave or ghetto which can mimic a large institution. People from institutions are generally relocated to their communities of origin, but some have ties elsewhere or nowhere. It may be more important to give them a home with friends. Homes may be provided by local authorities, charitable trusts, voluntary associations, or commercial agencies, but tenancies need careful working out, and management must be co-ordinated with the care and support services. Where there is an atmosphere of optimism, openness about intellectual impairment, and good support services, most parents will wish to care for their own sons and daughters at home at least into adulthood, but they do need a great deal of support. Their educational role with children is discussed below, but there are potential conflicts of interest later between their principle carer role and their own personal needs. This requires sensitive and skilled professional handling, and a range of services providing week-day activities outside the home, and a variety of respite care inside and outside the home.
Children and adults may be fostered or, in some countries, adopted; professional support is equally important in these situations. Some young adults will choose to leave the parental home early; others will continue to live with parents, but will eventually need a home of their own, and this should be anticipated and prepared well before the crisis of their parent’s death or incapacity. Professional support for parents can be supplemented by volunteers and community networks, not least those created by the parents themselves through their local associations.
An alternative model favoured by some is the ‘village community’, where people with intellectual impairment may experience freedom of movement, self-care, maximal personal decision-making, and a wide range of relationships and activities including appropriate work, but in a protected and separated ‘commune’. There is little integration or participation in other communities, and it shares some of the potential for ignorance and stigma with traditional institutions, but offers many elements of ‘ordinary life’ which may provide a satisfactory compromise for some adults.
Special needs for physical care can almost always be provided in ordinary home settings if staff are available, but some see a residual role for institutions in providing for those with serious behaviour disorders. These are few in any one community, and a linked network of small scattered units allows them to retain contact with their families and communities of origin. The most difficult are usually those with mild intellectual impairment, but there are no easy answers for seriously challenging behaviour, including those who commit serious offences.
Education and training
People with intellectual impairment have a right to education appropriate to their special needs. Early stimulation is of great benefit and can often be given in the home by parents. The Portage system and its many adaptations provide a model whereby parents are trained and assisted by a home visitor in teaching their own children, using developmental checklists, a special manual, and an inventory of parent teaching behaviours. Similar programmes are in use in many developed and developing countries and many translations are available. Other programmes use parent group workshops.
Special education for learning disabled children has made great strides in the last 25 years in most industrialized and many developing countries. Many school systems offer similar standards of provision as for other children, but separate schools are still common. The argument for integration into ordinary schools is not wholly won for those with severe learning disabilities, and many compromises exist. For more able children, special schools and special classes in ordinary schools are gradually being replaced by full or partial integration of individual children into ordinary classes, with additional resources as required. Where the quality of ordinary schools is very poor, it will be better to create special education of higher quality even if separate.
Young Down’s children may fit in relatively easily, although less so as they get older. Children with additional physical or behavioural problems pose serious difficulties for school staff, however well trained, resourced, and supported. Few school systems are willing to confront these problems on the scale of whole communities (as in Arrezzo, Italy). Children with autism, whether intellectually impaired or not, need special educational methods provided by specialist teachers.
Whatever schooling is created, it should be located within the culture of the community, and involve parents; the home remains a primary locus of learning. Wherever possible, parents should be partners with teachers and children in co-ordinated individual programmes. Such programmes need to extend beyond school leaving which is a stressful time for many. Further education and training in community colleges has become common in some countries, either as separate, specially designed courses or fully integrated. They include training in life-skills and leisure use, as well as vocational training, which may also be offered in schools.
Work and leisure
For many young mentally retarded people, vocational training is important to ease their way into ordinary work. But for those with severe intellectual impairment, an ‘ordinary working life’ in economic employment is not an option. However, they do need the dignity of work, a structure for each day, a satisfying occupation, and continuing education for their personal and social development. Occupational therapists are invaluable in directing the development of appropriate services. Sheltered, protected, or subsidized work situations are necessary; there are many examples of good practice, sometimes in ordinary workplaces.
Most, however, are day centres providing a variety of work, training, and leisure activities, and representing a daily focus for clients’ social life and contact with professional services. Where work is the primary focus, they should reflect the appearance and location of other workshops in the community. Work activities should not be merely repetitive and boring, but provide interest and scope for development. The best examples achieve a reputation for high-quality products which not only improves the economics of the centre, but gives a positive message to the community. This is easier for more able clients, who may be capable of ordinary productive work, but who may be the least favoured employees when general unemployment is high. In some communities, co-operatives of disabled and non-disabled workers are successful.
Special centres for work activities and environments should be supplemented by job placement programmes. Personal contact with employers, negotiated individual placement, and assistance with on-the-job training and supervision, can achieve higher levels of employment. In some countries subsidies are offered to employers to accept less efficient disabled workers. Others have legal employment quotas for disabled workers in larger firms, although these are seldom effective, especially for people with intellectual impairment.
Severely intellectually impaired people may need to be taught how to gain satisfaction from leisure pursuits, participate in community activities, and gain access to leisure facilities. They need encouragement, advocacy, and sometimes active help to face the many barriers to community participation. But given the opportunities, many will enjoy swimming, horse riding, boating, and other active sports, as well as more passive outings and holidays, and the ambience of places where people generally meet in their particular community. Many will respond to careful introduction into the local public house or bar, leisure centre, arts centre, or church.
Access to generic and specialist health services
One of the benefits of community services is that people with intellectual impairment can have ordinary access to primary health care and the specialists in general hospitals. However, in practice, this may need the mediation of mental retardation professionals to make it happen. Family doctors and their nurse colleagues can be invaluable resources for clients, families, teams, schools, and centres, as there are many health problems encountered by intellectually impaired people. Dental treatment often poses problems, and particular dentists need to gain special experience, and establish relationships with clients.
However, many also have mental, emotional, and behavioural problems, physical deformities, speech and sensory defects, epilepsy, cerebral palsy, and others which need specialist medical attention as for other people in their community. Thus psychiatrists, orthopaedic surgeons, ear and eye specialists, and neurologists will be particularly needed, together with psychologists, physiotherapists, and speech therapists. Although rights to specialist health care, and problems of access will vary within the health-care system of different countries, access will often have to be negotiated because of stigma, stereotyping, ignorance, and prejudice amongst hospital staff. In many service systems, specialists have formal attachments so that clients and families can get to know them, and they can also advise staff of day centres and homes. In the United Kingdom, the establishment of general psychiatrists ‘with a special interest’ in mental retardation, has been very successful.
Monitoring and evaluation
Co-ordinating training and development activities in a variety of service settings can be difficult. Many services are operated without clear, stated goals or the means of monitoring progress. In recent years, individual programme plans, based on systematic assessments, agreed with the client, family, and care staff, which are written down and monitored, have been widely adopted (Table 10). The idea draws elements from the ‘nursing process’, management ‘goal setting’, behaviour therapy, and task-centred social work traditions, and assumes a developmental approach, asserting that all intellectually impaired people have potential for development.

Table 10 Individual programme planning

Even where an individual programme plan system is very simple, it fosters discipline by using standard methods and records. It is especially needed where several staff members or centres concurrently serve one client; it should involve parents wherever possible. After assessment, precise short- and medium-term goals are agreed, trying to build on positive qualities and abilities, then the steps and tasks by which these objectives will be reached, and the people responsible for each one. Checklists may be used. The process is managed and supervised, and includes regular reviews, updating, and reassessment. It also allows ongoing evaluation of each client’s programme.
Much more demanding as in-house evaluation is the Program Analysis of Service System (Wolfensberger and Thomas 1983), and its more recent modifications. This provides a means of measuring local services against a normalization ideal, and formal training is offered to staff to enable them to apply it in their own work setting.
Evaluation of services can be approached in many ways. Comparative descriptive studies, quantitative or qualitative, interpreted by involved professionals, are not very informative. More useful, normative evaluation sets long-term outcome goals for each element of the service against which performance is measured; checklists are available. Formative evaluation has long-term goals but also sets short-term goals which are monitored regularly, so that services may be adjusted to maintain the overall direction. It is desirable to measure both effectiveness (achieving desired outcomes) and efficiency (relating effectiveness to costs) of the means by which services are delivered, but process measures must often be used as proxies for outcomes. Evaluation protocols, with resources, should be built into any new services, as with the NIMROD Project in South Wales (Davies et al. 1990). Heron and Myers’ thorough evaluation of services provided in the Sheffield Development Project is a rare example of a study of a complex service system (Heron and Myers 1983).
Accountability, including audit, is concerned with the proper use of resources and guaranteeing quality of care. Detailed costings are rarely available for specific elements of services where definitions may be very vague, and identifying discreet ‘units’ of care is not easy. Few cost–benefit studies have been done except for screening programmes. General agency audit tends towards oversimplification, but ‘clinical audit’ methods have greatly improved and have become generally accepted over the last few years. They should now be applied widely. It is difficult to take account of all cost elements, but potential benefits from the development of individual clients’ performance and thus lower levels of dependency, should not be forgotten. High-quality services may be a good investment! More evaluation research is needed if we are to learn from present experience how better to serve people with intellectual impairment in the future.
Conclusion and summary
Generalized intellectual impairment, learning disability, and dependency represent distinct but overlapping concepts and classifications within the field of mental retardation. Rigour and clarity of thinking are required if progress is to be made in our understanding of these phenomena. Severe intellectual impairment is usually coextensive with categories called severe mental handicap or retardation because of a common IQ definition (< 50).
Prevalence is dynamic and changing, subject to many factors affecting incidence and mortality of the many contributing organic syndromes. It is equally subject to changes in population denominators. Prevalence at birth appears to be decreasing in most developed countries, but prevalence in adults is generally increasing due to increased survival. The largest cohort is currently in their mid-thirties. Mild mental retardation is socially determined, and to understand it, we must focus mainly upon the processes and factors determining selection as ‘retarded’ in particular communities.
Many organic aetiological factors are implicated, especially where intellectual impairment is severe. Some syndromes we can now prevent. The cost of public health programmes will usually be justified because of the devastating consequencies of these disorders, and the costs of lifetime dependency.
Needs are individual and varied, and thorough assessment of physical impairments and disabilities can be standardized and individual programmes monitored. Services should be guided by the general principles of ‘normalization’ and the ‘ordinary life’ model of care, which accord intellectually impaired people the same basic rights and privileges as others, to adopt roles and undertake activities valued in their community and culture, neither isolated nor alienated from their peers and neighbours. However, they should not be invisible to social policy-makers, service planners, or the providers of resources, for they necessarily need positive discrimination and affirmative action.
There is much to overcome from the past. Although intellectually impaired people have not offended, they have been placed in custodial care; although they pose no threat to the community, they have been removed to remote institutions; although they are not sick, their lives have been controlled by doctors and nurses. This is all changing. Increasingly the need for a home, education, work, leisure, and medical care is being judged on criteria similar to those for the rest of the population, and provided by the same agencies and the same personnel, in the same place and in the same manner. They and their families are people first, with a range of cultural expectations not easy to fulfil if you are intellectually impaired. Professional objectives are to help them to fulfil those aspirations as much as possible.
Care in the community, using the ordinary life model, is expensive, but probably no more expensive than high-quality institutional care. Community-based services are more demanding for personnel, but potentially more satisfying. However, it is difficult to maintain the necessary commitment, zest, imagination, and sensitivity without the full support of senior professionals and service managers, satisfactory conditions of pay and employment, and appropriate training. The public health challenge is to ensure a full range of social services to promote satisfactory lives for people of all ages with intellectual impairment, and for their personal carers, in a comprehensive, community-based service, with access to specialist care as individually required. Establishing new structures, making them work, and undertaking rigorous evaluation are all part of the challenge that currently makes the field of mental retardation in public health particularly exciting.
Useful addresses
Many important publications on services for people with intellectual impairment are available from these groups.
American Association for Mental Deficiency (AAMD), Washington, DC, USA
British Institute of Learning Disabilities (BILD), Kidderminster, UK
Campaign for People with Learning Disabilities (CPLD), London, UK
Disability Office, World Health Organization, Geneva
Healthlink Worldwide (formerly Appropriate Health Resources Technology Action Group (HRTAG)) London, UK
Inclusion International (formerly the International League of Societies for People with Mental Handicap (ILSMH)) Brussels, Belgium
International Association for the Scientific Study of Mental Deficiency (IASSMD)
Joseph Rowntree Foundation, York, UK
King’s Fund Centre, London, UK
Lebenshilfe, Marburg, Germany
Mental Handicap in Wales Applied Research Unit, Cardiff, Wales, UK
National Institute for Mental Retardation (NIMR), Toronto, Canada
National Parents Associations
President’s Committee on Mental Retardation (PCMR), Washington, DC, USA
Royal MENCAP, London, UK
Socialstyren, Stockholm, Sweden
Baine, D. (1988). Handicapped children in developing countries. Faculty of Education, University of Alberta, Edmonton.
Breuning, S.E., Matson, J.L., and Barrett, R.P. (ed.) (1985). Advances in mental retardation and development disabilities. JAI Press, Greenwich, CT.
Clarke, A.M., Clarke, A.D.B., and Berg, J.M. (ed.) (1985). Mental deficiency: the changing outlook (4th edn). Methuen, London.
Cooper, B. (ed.) (1981). Assessing the handicaps and needs of mentally retarded children. Academic Press, London.
Current Opinion in Psychiatry; Mental Retardation Current Science, London. (Reviews published literature from the previous year. Mental Retardation section published annually (October issue).)
Dobbing, J. (ed.) (1984). Scientific Studies in Mental Retardation. Royal Society of Medicine/Macmillan, London.
Emoe, R. and Harmon, R. (1985). Continuities and discontinuities in development. Plenum, New York.
Fryers, T. (1996). Mental retardation in the developing world. In Psychiatry for the developing world (ed. D. Tantam, L. Appleby, and A. Duncan). Gaskell Press, London.
International Review of Research in Mental Retardation. Academic Press, San Diego, CA. (An almost annual publication of commissioned reviews.)
Janicki, M.P. and Wisnievsky, H.M. (1985). Aging and developmental disorders. Brookes, Baltimore, MD.
Marfo, K., Walker, S., and Charles, B. (ed.) (1986). Childhood disability in developing countries: issues in habilitation and special education. Praeger, New York.
Russell, O. (1997). Seminars in the psychiatry of learning disabilities. Gaskell Press, London.
Snowden, L.R. (ed.) (1982). Reaching the under-served: mental health needs of neglected populations. Sage, Beverly Hills, CA.
Belmont, L. (1984). International epidemiological studies of childhood disability: final report. Bishopp Becker Institute, Utrecht.
Durkin, M., Zaman, S., Thorburn, M., Hasan, M., and Davidson, L. (1991). Population-based studies of childhood disability in developing countries. International Journal of Mental Health, 20, 47–60.
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Overview of service provision: philosophy, law, planning, evaluation
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Elements of care
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King’s Fund (1984). An ordinary working life: vocational services for people with mental handicap. King’s Fund Centre, London.
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Raynes, N.V., Sumpton, R.C., and Flynn, M.C. (1987). Homes for mentally handicapped people. Tavistock, London.
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