Intellectual Disability (ID)

Intellectual Disability (ID) (also known as Intellectual Developmental Disorder, and previously Mental Retardation) is a disorder with onset during the developmental period that includes both intellectual and adaptive functioning deficits in 3 domains: (1) conceptual, (2) social, and (3) practical domains. Intellectual disability is a heterogeneous condition with many different etiologies. Two other diagnoses exist under the intellectual disability diagnostic category in the DSM-5: Global Developmental Delay and Unspecified Intellectual Disability.


Intellectual disability has a population prevalence of approximately 1%, and varies by age. Prevalence for severe intellectual disability is approximately 0.6%. Males are more likely than females to be diagnosed with both mild and severe forms of intellectual disability.[1]

The History of 'Mental Retardation'

Mental retardation was a diagnostic label that included the the categories of idiot, imbecile, and moron, based on early IQ testing data. All of these terms started as medical terms, and did not have the negative and pejorative connotations that they now have in popular culture. In 2010, President Barack Obama signed a bill known as Rosa's Law, which required the terms “mental retardation” and “mentally retarded” to be removed from federal use.[2] These terms were replaced with “intellectual disability” and “individual with an intellectual disability,” which is reflected by a change in the DSM-5 as well.
  • Schizophrenia is more common in individuals with ID, generally reported at 3-4%

Intellectual disability is a disorder with onset during the developmental period that includes both intellectual and adaptive functioning deficits in conceptual, social, and practical domains. The following 3 criteria must be met:

Criterion A

Deficits in intellectual functions, such as reasoning, problem solving, planning, abstract thinking, judgment, academic learning, and learning from experience, confirmed by both clinical assessment and individualized, standardized intelligence testing.

Criterion B

Deficits in adaptive functioning that result in failure to meet developmental and sociocultural standards for personal independence and social responsibility. Without ongoing support, the adaptive deficits limit functioning in one or more activities of daily life, such as communication, social participation, and independent living, across multiple environments, such as home, school, work, and community.

Criterion C

Onset of intellectual and adaptive deficits during the developmental period.

Severity Specifier

Specify whether:

  • Mild
  • Moderate
  • Severe
  • Profound

Global Developmental Delay is a diagnosis for individuals under the age of 5 years when the clinical severity level cannot be reliably assessed during early childhood. The diagnosis is given when an individual fails to meet expected developmental milestones in several areas of intellectual functioning, and applies to individuals who are unable to undergo systematic assessments of intellectual functioning, including children who are too young to participate in standardized testing. This category requires reassessment after a period of time.

Unspecified Intellectual Disability is a diagnosis for individuals over the age of 5 years when assessment of the degree of intellectual disability (intellectual developmental disorder) by means of locally available procedures is difficult or impossible because of sensory or physical impairments (like blindness, prelingual deafness, locomotor disability, presence of severe problem behaviors, or co-occurring mental disorder). This category should only be used in exceptional circumstances and requires reassessment after a period of time.

How is Intellectual Disability Actually Measured?

Intellectual functioning is usually measured with an individually administered and psychometrically valid, comprehensive, and culturally appropriate tests of intelligence. Individuals with intellectual disability have scores that are 2 standard deviations (or more) below the population mean. This includes a margin of error (generally ± 5 points).

For example, the Intelligence Quotient (IQ) has a mean of 100 and standard deviation of 15. This means a score of 65-75 (70 ± 5) would indicate intellectual disability. However, it is important to note that clinical training and judgment is required to accurately interpret this result, and to rule out other causes of this score.

Intellectual disability can be due to genetic syndromes at birth including: Down syndrome, Rett syndrome, and San Phillippo syndrome (among many others). Other times, it can be acquired due to an illness such as meningitis, encephalitis, or a traumatic brain injury during the developmental period. Prenatal causes include: inborn errors of metabolism, brain malformations, maternal disease or placental disease. Postnatal causes include hypoxic ischemic injury, infections, demyelinating disorders, seizure disorders (e.g. - infantile spasms), severe and chronic social deprivation, toxic metabolic syndromes, and heavy metal intoxications (e.g. - lead, mercury).

A diagnosis of intellectual disability should not be presumed simply because of a pre-existing genetic or medical condition. A differential diagnosis includes:

    • Intellectual disability is categorized as a neurodevelopmental disorder and is distinct from the neurocognitive disorders, which are characterized by a loss of cognitive functioning. Major neurocognitive disorder may co-occur with intellectual disability (e.g. - an individual with Down syndrome who develops Alzheimer's disease, or an individual with intellectual disability who loses further cognitive capacity following a head injury). In such cases, the diagnoses of intellectual disability and neurocognitive disorder may both be given.
    • These neurodevelopmental disorders are specific to the communication and learning domains and do not show deficits in intellectual and adaptive behaviour. They may co-occur with intellectual disability. Both diagnoses are made if full criteria are met for intellectual disability and a communication disorder or specific learning disorder.
    • Intellectual disability is common among individuals with autism spectrum disorder. Assessment of intellectual ability may be complicated by social-communication and behaviour deficits inherent to autism spectrum disorder, which may interfere with understanding and complying with test procedures. Appropriate assessment of intellectual functioning in autism spectrum disorder is essential, with reassessment across the developmental period, because IQ scores in autism spectrum disorder may be unstable, particularly in early childhood.

A comprehensive work up for intellectual disability includes assessing intellectual capacity and adaptive functioning, identifying genetic, non-genetic, and associated medical conditions (such as cerebral palsy and seizure disorders). A prenatal/perinatal history, family pedigree, physical examination, genetic evaluation (karyotype, chromosomal microarray, and/or genetic syndrome tests), metabolic screening, and neuroimaging assessment can also be important investigations.

Medications, particularly antipsychotics, may be used to manage challenging behaviour such as aggression in individuals with intellectual disability. When the behaviour does not arise from an underlying mental illness, this is off-label use and evidence of efficacy is very poor.[3] Furthermore, these patients face higher risks of drug-related side effects.

For Healthcare Providers
1) American Psychiatric Association. (2013). Diagnostic and Statistical Manual of Mental Disorders (5th ed.). Arlington, VA.