Intellectual Disability, formerly known as “Mental Retardation,” is a disorder with onset during the developmental period. It includes intellectual deficits and difficulty functioning in daily life in areas such as communication, self-care, home living, social/interpersonal skills, self-direction, academics, work, leisure, health, and safety.

Intellectual disability has many different etiologies and may be seen as a final common pathway of various pathological processes that affect the central nervous system.

Prior to the publication of DSM-5 in 2013, diagnostic criteria for Mental Retardation required that an individual score ~2 standard deviations below their expected IQ compared to same-age-peers on standardized IQ tests (Full Scale Intellectual Quotient ≤ 70).

In the DSM-5, IQ scores have been deemphasized. There is no longer a “cut-off” score or threshold per se for establishing a diagnosis. Rather, scaled IQ scores are evaluated in context of individual’s entire “clinical picture.”

The rationale for this change was that while scaled IQ scores represent approximations of conceptual functioning, they may be insufficient to assess reasoning in real-life situations and mastery of practical tasks within conceptual, social, and practical domains. For example, a person with an IQ score above 70 may have such severe adaptive behaviour problems in social judgment, social understanding, and other areas of adaptive functioning that the person’s actual functioning is comparable to that of individuals with a lower IQ score. For this reason, clinical judgment is required to interpret the results of an IQ test.