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 su icidal thoughts is essential in the assessment process. Be-
cause of a lack of awareness of risk and dang er, accidental injury rates may be increased.
Prevalence
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.
Development and Course
Onset of intellectual disability is in the de velopmental 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 identifi able 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 appare nt. All criteria (in-
cluding Criterion C) must be fulfilled by hi story or current presentation. Some children
under age 5 years whose presentation will even tually meet criteria for intellectual disabil-
ity have deficits that meet criter ia 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 syndro me). 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-