in intellectual disability, with rates of some conditions (e.g., ment al disorders, cerebral
palsy, and epilepsy) three to fo ur times higher than in the ge neral population. The prognosis
and outcome of co-occurring diagnoses may be influenced by the presence of intellectual
disability. Assessment procedur es may require modifications because of associated disor-
ders, including communication disorders, autism spectrum disorder, and motor, sensory,
or other disorders. Knowledgeable informan ts are essential for identifying symptoms
such as irritability, mood dysregulation, aggression, eating problems, and sleep problems,
and for assessing adaptive functioning in various community settings.
The most common co-occurring mental an d neurodevelopmental disorders are atten-
tion-deficit/hyperactivity disorder; depressi ve and bipolar disorders; anxiety disorders;
autism spectrum disorder; stereotypic movement d isorder (with or without self-injurious
behavior); impulse-control disorders; and ma jor neurocognitive disorder. Major depres-
sive disorder may occur througho ut the range of severity of intellectual disability. Self-
injurious behavior requires pr ompt diagnostic attention and may warrant a separate di-
agnosis of stereotypic movement  disorder. Individuals with intellectual disability, partic-
ularly those with more severe intellectual disability, may also exhibit aggression and
disruptive behaviors, including harm of others or property destruction.
Relationship to Ot her Classifications
ICD-11 (in development at the time of  this publication) uses the term intellectual develop-
mental disorders  to indicate that these are disorders that involve impaired brain functioning
early in life. These disorders are described in  ICD-11 as a metasyndrome occurring in the
developmental period analogous to dementia or neurocognitive disorder in later life.