Intellectual Disability (Intellectual Developmental Disorder) 37
Diagnostic Features
The essential features of intellectual disab ility (intellectual developmental disorder) are
deficits in general mental abilities (Criteri on A) and impairment in everyday adaptive
functioning, in comparison to an individual ’s age-, gender-, and socioculturally matched
peers (Criterion B). Onset is during the developmental period  (Criterion C). The diagnosis
of intellectual disability is based on both clinical assessment and standardized testing of
intellectual and adaptive functions.
Criterion A refers to intellectual function s that involve reasoning, problem solving,
planning, abstract thinking, judgment, lear ning from instruction and experience, and
practical understanding. Critical componen ts include verbal comprehension, working
memory, perceptual reasoning, quantitative reasoning, abstract thought, and cognitive ef-
ficacy. Intellectual functioning is typically me asured with individually administered and
psychometrically valid, comprehensive, cultur ally appropriate, psychometrically sound
tests of intelligence. Individuals with intellec tual disability have scores of approximately
two standard deviations or more below the population mean, including a margin for mea-
surement error (generally +5 points). On te sts with a standard deviation of 15 and a mean
of 100, this involves a score of 65–75 (70 ± 5). Clinical training and judgment are required
to interpret test results and assess intellectual performance.
Factors that may affect test scores include practice effects and the “Flynn effect’ (i.e.,
overly high scores due to out- of-date test norms). Invalid scores may result from the use of
brief intelligence screening tests or group tests;  highly discrepant individual subtest scores
may make an overall IQ score invalid. Instrume nts must be normed for the individual’s so-