40 Neurodevelopmental Disorders
genetic or medical condition. A genetic syndro me linked to intellec tual disability should
be noted as a concurrent diagnosi s with the intellectual disability.
Major and mild neurocognitive disorders. Intellectual disability is categorized as a neu-
rodevelopmental disorder and is distinct from the neurocognitive disorders, which are
characterized by a loss of cognitive function ing. 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 cogni-
tive capacity following a head injury). In such cases, the diagnoses of intellectual disability
and neurocognitive disorder may both be given.
Communication disorders and specific learning disorder. These neurodevelopmental
disorders are specific to the communication and learning domains and do not show defi-
cits in intellectual an d adaptive behavior. They may co-o ccur with intellectual disability.
Both diagnoses are made if full criteria are met for intellectual disability and a communi-
cation disorder or specific learning disorder.
Autism spectrum disorder. Intellectual disability is common among individuals with
autism spectrum disorder. Assessment of intellectual ability may be complicated by so-
cial-communication and behavior deficits inherent to auti sm spectrum disorder, which
may interfere with understanding and comply ing with test procedures. Appropriate as-
sessment of intellectual functioning in autism spectrum disorder is essential, with reas-
sessment across the developmental period, becaus e IQ scores in auti sm spectrum disorder
may be unstable, particularly in early childhood.
Comorbidity
Co-occurring mental, neurodevelopmental, me dical, and physical conditions are frequent
in intellectual disability, with rates of some conditions (e.g., ment al disorders, cerebral