Epidemiology is essentially the study of a disorder in a given population. Knowledge of the distribution of a disorder in a population can increase understanding of the causes and how best to manage it.

Vee P Prasher (UK) and Giri P Madhavan (UK)

Basic principles

Epidemiology is essentially the study of a disorder in a given population.  Knowledge of the distribution of a disorder in a population can increase understanding of the causes and how best to manage it.  Epidemiological research may investigate:

  • the frequency of a disease
  • the causes of a disease
  • the natural history of a disease
  • the characteristics of a disease, influenced by such factors as age, sex, social class and ethnic background.

Research may be used to test the efficacy and efficiency of healthcare intervention to prevent or treat the disorder.  It may also assist in healthcare planning. Figure 1 lists commonly used measures of disease frequency.




Point prevalence rate

Refers to the proportion of people in a defined population who are affected by the disorder at a given point in time.

Period prevalence rate

Proportion of people who are affected by a disorder at any time within a stated period.

Incidence rate

Measure of new episodes of illness: the proportion of formerly well subjects who developed an illness in a defined period of time (usually one year)

Relative Risk (RR)

The ratio of the incidence of an outcome in those that are exposed to a certain risk factor compared to the incidence in an unexposed group

Odds Ratio (OR)

The ratio of the odds of disease in exposed individuals relative to the unexposed

Number needed to treat (NNT)

Meaningful way of expressing the benefit of any intervention: relates to how many individuals need to be treated for one individual to benefit

Epidemiological strategies

There are a number of general epidemiological strategies which are important in terms of epidemiological research.

Case definition - This is a principal issue in epidemiological research.  Although it may appear straightforward, there are a number of issues relating to the questions 'what is a case?' or 'how is any given illness defined?'  In psychiatry, this is an important issue, particularly when many disorders are continuous rather than discrete phenomena. For example, in people with intellectual disabilities considerable uncertainty remains regarding the valid detection of autism or dementia.  For a meaningful conclusion to be drawn, there must be uniformity in definitions of a given disease.  Also, if aetiological factors are to be investigated, what constitutes a given case must be agreed.  To this end a number of classification systems have been developed, principally ICD-10 (WHO, 1992), DSM-V (APA, 2013) and DM-ID (2007).

Sampling methods used - It is usually not possible to examine the entire population; instead, a sample subset of the population is investigated. However, it is important that the sample represents the larger population in an unbiased fashion.  For example, investigating the prevalence of an illness in people with severe intellectual disability prevents the conclusions being generalized to people who may have mild or moderate intellectual disability.

There are a number of sampling techniques, including:

  • Simple Random Sampling
  • Stratified Random Sampling
  • Systematic Random Sampling
  • Multistage Random Sampling
  • Multiphase Sampling
  • Cluster Sampling
  • Non-probability Sampling

Sample size - Research proposals often focus on the practicalities of studies and do not place as much importance as they should on the sample size of the study.  The sample size will affect whether the original hypotheses can be tested significantly, and the researcher should undertake a power calculation before any study is begun to determine what sample size is needed to answer their hypotheses.

Standardization - To be able to compare and contrast rates of any illness between areas or groups, it is important that underlying confounding factors are controlled for, in particular, age, sex, social class and severity of intellectual disability.

Investigatory studies - There are a number of methods of study in epidemiology, and these may involve observational or interventional studies.  They may be restrictive, whereby the pattern of disease in a population is described, or an analytical planned investigation to test a hypothesis may be undertaken.  Further, studies may be retrospective or prospective.  In retrospective studies the researcher examines past events of experiences; prospective studies involve collecting new data.  There are four main epidemiological research study types, which are listed in Figure 2.


Cross-sectional study

Used to measure the prevalence of an illness or event.  Observational and descriptive. A single measurement at one moment in time.  Results usually limited by the study's inability to identify cause or relationship

Case-control study

Usually involves two groups: one group of people with the disease and another unaffected (control) group.  The relationship of aetiological factors for the disease can then be examined by comparing measures between the groups.  Matching of underlying variables is an important issue. Selection of subjects needs to be unbiased

Cohort study

A defined group of individuals is studied, usually over a defined period of time, to ascertain the frequency with which selected characteristics change or develop.  Can be both prospective and retrospective.  Often used to ascertain the effect of exposure to particular hazards. Can be time-consuming and expensive, and can rapidly become out of date

Controlled clinical trial

Intervention studies that are usually prospective and experimental. Aims to determine the effects of an intervention or therapeutic measure.  Two groups must be matched prior to any intervention: one group is given the treatment and the other usually the placebo.  Groups are followed up over time and compared on a number of given measures

Specific issues in intellectual disability

Definition of caseness - How applicable accepted standardized diagnostic criteria (ICD-10, DSM-V, DM-ID) are for people with learning [intellectual] disability has still not been fully researched.  Although in general they are applicable, caution may be needed when using such criteria.  For example, when investigating schizophrenia, some of the requirements for diagnosis depend on good verbal communication and an ability to verbalize intellectual thoughts and perceptual changes.  This requires a reasonable degree of underlying intelligence, which may not be present in all people with learning disability  (DM-ID, 2007).

Difficulties in diagnosis - Standardized diagnostic assessments and tools are not readily available for the intellectual disability population.  Few have been widely accepted or have good validity and reliability.  Often the diagnostic assessment is a clinical one and therefore subject to a high degree of interview bias.

Behaviour-mental illness dichotomy - Further investigation is required as to whether presenting symptoms are those of a mental illness or part of long-standing behavioural change.  Such behaviour may be secondary to an environmental situation, and may be a reaction to a life event rather than an actual psychiatric illness.

Target population - There is a large degree of heterogeneity in intellectual disability, with considerable variation in underlying intellectual functioning, communication skills, adaptive behaviour, associated mental disorders, and in the different causes of disability.  Findings from any given study may not be directly representative of the target population.

Sample size - Often the number of people with intellectual disability available to participate in studies is limited.  Once a number of issues (e.g. ethics, consent/assent, carer agreement) have been addressed the number of individuals recruited may be smaller still, leading to a failure to meet the sample size suggested by the power calculation.  Therefore, the conclusions reached may have to be treated with caution.

Direct/indirect studies - Owing to limitations of communication and intellectual impairment, subjects often cannot participate directly in studies.  The vast majority of studies are thus observational and give limited information.

Specific epidemiological studies

Terminology: Many terms and definitions are used to refer to intellectual disabilities, such as mental retardation, mental handicap, intellectual disabilities, and learning disabilities.  According to WHO, the use of terminology varies between countries with the term mental retardation being the most used (76.0%), followed by intellectual disabilities (56.8%), mental handicap (39.7%) and mental disability (39.0%).  Use of the term intellectual disability seemed to vary with level of income and time period (Schalok et.al (2007)).  The WHO defines intellectual disability as “a significantly reduced ability to understand new or complex information and to learn and apply new skills (impaired intelligence).  This results in a reduced ability to cope independently (impaired social functioning) and begins before adulthood, with a lasting effect on development”.

Prevalence and aetiology: Prevalence rates vary depending on the study design and the population studied, but according to the World Health Organization the true prevalence of intellectual disability is close to 3%. Maulik et al (2011) undertook a meta-analysis of population-based prevalence studies and reported that given “a paucity of research in areas of incidence and mortality, studies reporting on prevalence assume greater significance as sources of critical data for estimating the burden of intellectual disability”.  Many studies were hampered by imperfections in study design, and estimates of prevalence rates.  Individuals with mild disability represent the largest proportion (approximately 2.5% of the whole population); moderate intellectual disability involves approximately 0.4% of the population, and severe and profound levels combined account for approximately 0.1% (see also Cooke 2003).

Epidemiological studies have been undertaken looking at the causes of intellectual disability, including demographic, parental and environmental factors.  Down’s syndrome, for example, occurs at the same rate in all populations regardless of race, geographical location or season of birth.  The principal association appears to be that of an increased rate with increasing maternal age.  A range of environmental factors have been studied, including fluoride in drinking water, radiation and thyroid dysfunction in mothers, but generally there is no evidence supporting an environmental agent as a causative factor for Down’s syndrome.

Mortality rates: Heslop et al (2015) comment on the difficulty in accurate determination of mortality data in people with intellectual disabilities.  This stems from combinations of lack in consistency in identifying those with intellectual disabilities, multiple coding options for intellectual disabilities, incomplete registers, lack of recording of intellectual disability on death certification as well as coding errors on such certificates. Day and Strauss (2005) investigated mortality rates in a large population of people with intellectual disabilities and concluded that mortality rates did not differ by sex and was consistent with their previous work (Strauss and Eyman (1996)).  Up to the age of 35 years, mortality rates for people with Down’s syndrome were comparable with those for people with intellectual disability due to other causes.  Subsequently, however, the mortality rates for individuals with Down’s syndrome doubled every 6.4 years, compared with 9.6 years for people without Down’s syndrome.  Frid et al (1999) investigated mortality in Down’s syndrome in relation to the presence or absence of congenital malformations, such as heart lesions; mortality rates within 10 years of the birth were 40.6% and 23.5% respectively.  The presence of gastrointestinal malformation further increased the risk of mortality.  Common causes of death included congenital abnormalities, neonatal complications and respiratory infections. McGuigan et al (1995) confirmed that the age-specific standardized mortality rates for people with intellectual disabilities are often higher than in the general population.  This applies to both men and women.  More recently (Heslop et al 2013) reported on, the Confidential Inquiry into Premature Deaths of People with Learning Disabilities (CIPOLD) which demonstrated that men with intellectual disabilities died on average 13 years earlier than in the general population, while women with intellectual disabilities died 20 years earlier.  Hosking et al (2016) concluded that a total of 37% of all deaths among adults with intellectual disability were classified as being amenable to health care intervention compared with 22.5% in the general population.

In a cause specific mortality study by Tyrer et al (2009) in moderate to profound intellectual disabilities, a total of 503 (17% of population) adults with intellectual disability died during the 14-year study period.  Relatively high cause-specific mortality was seen for deaths caused by congenital abnormalities (cause-specific standardized mortality ratios SMR = 8560), diseases of the nervous system and sense organs (SMR = 1630), mental disorders (other than dementia) (SMR = 1141) and bronchopneumonia (SMR = 647).  Excess deaths were also seen for diseases of the genitourinary system or digestive system, cerebrovascular disease, other respiratory infections, dementia (in men only), other circulatory system diseases (in women only) and accidental deaths (in women only). Two-fifths of deaths recorded in adults with intellectual disability mentioned intellectual disability or an associated condition as a contributing cause of death.

Prevalence studies of psychiatric disorders: A number of studies have investigated psychiatric disorders among adults with intellectual disability (e.g. Cooper and Bailey, 2001; Haveman et al, 1989); these have generally been point prevalence studies. Some mental health conditions appear to be more common among those with intellectual disabilities, although there remain challenges with accurate prevalence estimates, notably with healthcare providers having difficulty with diagnosing mental disorders in those with intellectual disability (Kerker et al. 2004).

  • Hughes-McCormack et al (2017) have recently published the first whole country study and discovered for those with intellectual disabilities, 12.8% children, 23.4% adults and 27.2% older adults had mental health conditions compared with 0.3,5.3 and 4.5% of the general population.
  • Cooper and Bailey (2001) assessed 207 adults with intellectual disability, and found a psychiatric disorder rate of 49.2%. Adults with more severe intellectual disability had higher rates of additional psychiatric disorders.
  • In contrast, Crews et al (1994) investigated the prevalence of psychiatric disorders in a residential population (n=1273).  Individuals were aged 10-80 years, with a mean age of 40 years. The point prevalence rate of diagnosis based on DSM-III-R criteria was 15.6%.  Psychiatric diagnoses were more likely in individuals with mild retardation.  Affective disorder was the most common, followed by psychotic disorder.
  • Prasher (1995) investigated psychiatric disorders in 201 adults with Down's syndrome aged 16 years and over: the rate of psychiatric disorder was 28.9%.  The commonest disorders were dementia of Alzheimer's type, depression, conduct disorders, and obsessive-compulsive disorders (see also Prasher 2003).
  • Altarac et al. (2007) showed that the lifetime prevalence of intellectual disability in US children is 9.7%.  Although prevalence of intellectual disability is lower among average developing children (5.4%), it still affected 2.7 million children compared with 3.3 million (27.8%) children with special healthcare needs.
  • According to Morgan et al (2008), 31.7% of people with an intellectual disability had a psychiatric disorder (n=245,749); 1.8% of people with a psychiatric illness had an intellectual disability.  Schizophrenia was greatly over-represented among individuals with a dual diagnosis.  Pervasive developmental disorder was more common among people with a dual diagnosis than among individuals with intellectual disability alone.  Down’s syndrome was much less prevalent among individuals with a dual diagnosis despite being the most predominant cause of intellectual disability.  Individuals with a dual diagnosis had higher mortality rates and were more disabled than those with psychiatric illness alone.
  • Bhaumik et al (2008) studied 2,711 adults with intellectual disability between 2001 and 2006.  The prevalence of psychiatric disorders among the total study population was 33.8%; the most common disorders were behaviour disorder (19.8%) and autistic spectrum disorders (8.8%).  Epilepsy was highly prevalent (60.8%) among those attending psychiatric services without a mental health diagnosis.  Behaviour disorders and autistic spectrum disorders were more common in men and in adults with severe/profound intellectual disability, whereas schizophrenia and organic disorders were more common in women and in adults with mild/moderate intellectual disability.  Depression was also more common in women with intellectual disability.
  • 4,337 children aged 4/5 years were studied and followed up at age 6/7 years by Emerson et al (2010). When compared to typically developing children, children identified at age 4/5 years as having intellectual disability or borderline intellectual functioning showed significantly higher rates of possible mental health problems for total difficulties and on all five SDQ subscales at age 6/7 years (OR 1.98-5.58) and were significantly more likely to be exposed to socio-economic disadvantage at age 4/5 and 6/7 years.

Prevalence studies of physical disorders: A number of studies have looked at the prevalence of physical health problems in people with intellectual disability (e.g. Gentile et al (2015), Day and Jancar, (1994); Hand, (1994).  As with psychiatric studies, individuals with intellectual disability have been recruited by different means and undergone assessments that at times have not been defined; furthermore, the studies have usually reported point prevalence.  Hand (1994) found that epilepsy, cerebral palsy and neurological impairment were the commonest conditions.  Forty-two per cent of those studied reported having no major or chronic physical problems.  In ageing adults with intellectual disabilities, common health problems include:

  • hearing and visual impairment
  • mobility problems
  • heart conditions
  • diabetes
  • fractures and osteoporosis.

Some of the physical disorder statistics, according Emerson et al. (2016) can be seen in table 1



2% vs 1% in general population



Respiratory disorder

1% vs 1% in general population


42% vs 26%


8% vs 4%


2% vs 1%

Multiple morbidity

8% vs 4%



Evidence from epidemiological studies suggests that people with intellectual disability are more susceptible to mental and physical illnesses than the general population.  The reasons for this include biological risk factors (such as genetic abnormalities and brain damage) and psychological risk factors (such as stigmatization and impaired social integration).  However, it is important to recognize the heterogeneity of this population and the inherent difficulties in conducting high-quality epidemiological research with people with intellectual disabilities when data is both sparse and variable in quality.

Although interest is growing in the field of epidemiology in intellectual disability, a number of concerns remain regarding epidemiological methods. Several important issues require further investigation, including:

  • the incidence of physical disorders in the people with intellectual disabilities population
  • therapeutic intervention studies
  • the causes of physical health morbidity
  • the impact of gender
  • recording of and access to high quality data
  • the impact of age on morbidity (with improvements in life expectancy).


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First published in Psychiatry, Volume 2:8, August 2003 and reprinted with the kind permission of The Medicine Publishing Company.

This article was updated in 2011, and further updated and revised in 2017.