ical judgment. When standardized testing is difficult or impossible, because of a variety of factors (e.g., sensory impairment, severe problem behavior), the individual may be diag- nosed with unspecified intellectual disability. Adaptive functioning may be difficult to assess in a controlled setting (e.g., prisons, detention centers); if possible, corroborative in- formation reﬂecting functioning outside those settings should be obtained.
Criterion B is met when at least one domain of adaptive functioning—conceptual, so- cial, or practical—is sufficiently impaired that ongoing support is needed in order for the person to perform adequately in one or more life settings at school, at work, at home, or in the community. To meet diagnostic criteria for intellectual disability, the deficits in adap- tive functioning must be directly related to the intellectual impairments described in Cri- terion A. Criterion C, onset during the developmental period, refers to recognition that intellectual and adaptive deficits are present during childhood or adolescence.
Intellectual disability is a heterogeneous condition with multiple causes. There may be associated difficulties with social judgment; assessment of risk; self—management of behav- ior, emotions, or interpersonal relationships; or motivation in school or work environments.
Lack of communication skills may predispose to disruptive and aggressive behaviors. Gull- ibility is often a feature, involving naiveté in social situations and a tendency for being easily led by others. Gullibility and lack of awareness of risk may result in exploitation by others and possible victimization, fraud, unintentional criminal involvement, false confessions, and risk for physical and sexual abuse. These associated features can be important in crim- inal cases, including Atkins-type hearings involving the death penalty.
Individuals with a diagnosis of intellectual disability with co-occurring mental disor- ders are at risk for suicide. They think about suicide, make suicide attempts, and may die from them. Thus, screening for suicidal thoughts is essential in the assessment process. Be- cause of a lack of awareness of risk and danger, accidental injury rates may be increased.
Intellectual disability has an overall general population prevalence of approximately 1%, and prevalence rates vary by age. Prevalence for severe intellectual disability is approxi- mately 6 per 1,000.
Onset of intellectual disability is in the developmental period. The age and characteristic features at onset depend on the etiology and severity of brain dysfunction. Delayed motor, language, and social milestones may be identifiable within the first 2 years of life among those with more severe intellectual disability, while mild levels may not be identifiable un- til school age when difficulty with academic learning becomes apparent. All criteria (in- cluding Criterion C) must be fulfilled by history or current presentation. Some children ity have deficits that meet criteria for global developmental delay.
When intellectual disability is associated with a genetic syndrome, there may be a char- acteristic physical appearance (as in, e.g., Down syndrome). Some syndromes have a behavioral phenotype, which refers to specific behaviors that are characteristic of particular genetic disorder (e.g., Lesch—Nyhan syndrome). In acquired forms, the onset may be abrupt following an illness such as meningitis or encephalitis or head trauma occurring during the developmental period. When intellectual disability results from a loss of pre- viously acquired cognitive skills, as in severe traumatic brain injury, the diagnoses of in- tellectual disability and of a neurocognitive disorder may both be assigned.