ICD-10 and DSM-IV-TR definitions of intellectual disabilities are similar:

IQ is a continuous measure, so when basing a definition of intellectual disabilities upon it the threshold is arbitrary, and changes in threshold can have a large impact on prevalence. For example, in the past the American Association on Intellectual and Developmental Disabilities’ criteria had an IQ threshold of 84. This was subsequently changed to 70 in 1973, and then to 70-75 in 1992. ICD-10 and DSM-IV-TR definitions are not statistical constructions; the requirement of impaired adaptive functioning may half estimated prevalence rates compared with a statistical definition. Furthermore, different methods to assess intelligence and adaptive behaviour can lead to different prevalence rates.

Prevalence of intellectual disabilities is reported to be much higher in developing than developed countries, due to socio-economic factors, although mild intellectual disabilities may possibly be less disadvantaging in non-literate societies. Iodine deficiency is the most common preventable cause of intellectual disabilities worldwide, and is indigenous in some regions of Asia and Africa. Exposure of populations to heavy metals and toxins can lower the average population IQ by a few points, hence some people who would otherwise have had low-average ability move into the intellectual disabilities range. Regions with a high level of consanguineous marriages can also have higher prevalence. For example, Tay-Sachs disease is prevalent amongst Ashkenazi communities, although premarital genetic counselling has markedly reduced it in the United States. The availability and sophistication of antenatal, perinatal, and neonatal care account for some differences between countries. Some studies show differences in ethnic groups, although the cultural suitability of the measures used may have contributed to these findings.

Prevalence varies with time, due to preventative measures, and social developments. In developed countries, an increase in prevalence was seen in the early 1960s, with falling prevalence thereafter, due to developments in neonatal care with increasing survival of very low birth weight infants. Down syndrome is the most common chromosomal disorder causing intellectual disabilities, and survival rates of neonates and children with Down syndrome have increased substantially in recent decades, primarily due to access to surgery for congenital heart disease. The widespread introduction of antenatal screening for Down syndrome might have been expected to reduce the population prevalence of Down syndrome through lowering birth rate, but rising maternal age at birth, and increasing life expectation counter this, and there appears to be little change in the population prevalence. The widespread introduction of antenatal screening for phenylketonuria in the 1960s, and congenital hypothyroidism in the 1970s has virtually eliminated intellectual disabilities due to these conditions in
developed countries. Better living conditions, with individualized packages of support, and a political agenda for social justice and equality of access to health care and supports may all have contributed to the increasing life expectation for people with intellectual disabilities, although this is still lower than that of the general population. Increasing maternal age and increasing maternal alcohol consumption are expected to lead to higher birth rate of infants with foetal alcohol syndrome, and genetic causes of intellectual disabilities. Overall, there appears to have been little change in prevalence compared with 50 years ago.

Prevalence is higher in child than adult cohorts, and lower in older than younger adult cohorts, with the highest prevalence at around age 10 years. This is due to intellectual disabilities having been identified by this age, combined with an earlier age of death for persons with intellectual disabilities compared with the general population. Children with the mildest intellectual disabilities are likely to benefit from additional support for learning at school, but will develop skills and experience over time, such that some no longer meet criteria for intellectual disabilities in adulthood.

Reported prevalence varies with the methods of population ascertainment. For children, the ascertained prevalence doubled compared with case registers, when record linkage to education department data on educational attainments was included (giving an estimated prevalence of 1.4 per cent).⁽³⁾ There was a disproportional increase in indigenous Australian children, who were possibly false positives. For adult populations, only a proportion with intellectual disabilities will be in contact with specialist health services for adults with intellectual disabilities. Ascertainment is higher when data is combined from primary health care, specialist health services, and social services, if the provision of day opportunities, supported work, respite care, funded support packages, and direct payments is considered. This is likely to identify almost all persons with moderate to profound intellectual disabilities, and adults with mild learning disabilities receiving support; it will not identify adults with IQ below 70 who no longer have impaired adaptive functioning (who therefore do not meet ICD-10 or DSM-IV-TR criteria for intellectual disabilities) and do not need support, or some people who receive all their support exclusively from unpaid carers. The assessment of IQ (culturally sensitive), adaptive functioning and support needs, plus medical assessment, of all individuals within a whole population or a representative sample would provide accurate prevalence data for that time point and area. However, this would be a substantial undertaking, in view of the tens or hundreds of thousands of participants required.

There have been many studies of prevalence of intellectual disabilities. For the reasons above, there is substantial variation in reported prevalence in developed countries, varying from 2 to 85/1000 general population, and there are few robust studies in developing countries. Less variability is found between studies of moderate to profound intellectual disabilities. Given the variation, it is inappropriate to provide average figures from across the studies. Interpreting the literature, we suggest that prevalence of intellectual disabilities in the United Kingdom may be in the order of 9-14/1000 childhood population, and 3-8/1000 adult population, varying with time and geography. However, it should be noted that the figure of 2 per cent is frequently assumed. Intellectual disabilities are more prevalent in males than females, particularly amongst children, young- and middle-aged adults: the reported ratio varies between 1:1 and 2:1. At older age, the gender ratio equalizes due to greater life expectancy of women compared with men (mirroring the general population), and at extreme old age, women may even outnumber men. The distribution of level of intellectual disabilities varies with age, due to the shorter life expectancy of people with more severe intellectual disabilities. Mild intellectual disabilities are associated with socio-economic status. These issues are explored in greater depth elsewhere.^(1,2)

Mental ill-health is thought to be commonly experienced by adults with intellectual disabilities. Many of the existing prevalence studies have methodological limitations, accounting for the wide discrepancy in reported prevalence which ranges from 7 to 97 per cent. Limitations have included biased sampling; reliance upon existing case-note information, or instruments designed as screening tools only; lack of information on the extent of detail within assessments, the instruments, or diagnostic criteria used; and population-based studies limited by small cohort sizes. Other limitations include failure to indicate whether rates are lifetime, point, or period prevalence; reporting combined prevalence for children and adults; reporting mental ill-health in total, but not describing nor being comprehensive as to what is, and what is not, included (particularly with regards to problem behaviours, autistic spectrum disorders, attention-deficit hyperactivity disorder, and anxiety disorders); and studying selected subgroups such as only adults with verbal communication skills. All of these points must be carefully considered when interpreting and drawing conclusions from the existing literature.

Prevalence of mental ill-health varies, depending upon the diagnostic criteria employed. This is because many of the diagnostic categories within The ICD-10 Classification of Mental and Behavioural Disorders: Diagnostic Criteria for Research (DCR), and the DSM-IV-TR contain criteria that cannot be met due to the person’s degree of intellectual disabilities and communication skills, and do not include other criteria that are important in this population. For these reasons, Diagnostic Criteria for Psychiatric Disorders for Use with Adults with Learning Disabilities/Mental Retardation (DC-LD) was developed for use specifically with this population. These, and
other important diagnostic issues are explored in further depth elsewhere.⁽⁴⁾

Population-based studies where participants received a psychiatric assessment are shown in the Table 10.2.1. ^(5,6,7,8,9) A high prevalence of mental ill-health was reported in all but Lund’s study, which used assessment methods which would today be considered limited.⁽⁶⁾ Point prevalence is higher than that observed in the UK general population. Specific types of mental ill-health with a higher prevalence compared with the general population including problem behaviours, autism, dementia, bipolar disorder, and psychoses. Dementia is part of the behavioural phenotype of Down syndrome, and also occurs three to four times more commonly amongst people with intellectual disabilities of other causes.^(10,11) Bipolar disorder occurs at about double the prevalence of that reported for the general population. This is despite a high proportion of people (about 25 per cent) taking mood-stabilizing drugs (typically for epilepsy management). Depression is either more prevalent, or occurs at the same rate, depending upon the criteria used. Prevalence of non-affective psychotic disorders (including schizoaffective disorders) has consistently been reported to be higher than for the general population; a recent study found a point prevalence of 4.4 per cent including schizophrenia, in remission, or 4.0 per cent for psychosis, currently in episode. Problem behaviour is the most prevalent type of mental ill-health at 22.5 per cent (Consultant psychiatrist’s opinion) or 18.7 per cent (DC-LD) in the most recent of the studies.⁽⁹⁾