This category is made up of more than 42 different conditions and illnesses, all of which usually first appear in infancy, childhood, or adolescence. It includes mental retardation, for which two diagnostic signs are below-average intellectual functioning (IQ less than 70) and inability to meet standards of self sufficiency that are normal for an age group.

Retardation is further qualified by different levels of severity, from mild (55 IQ) to profound (below 20 IQ). Other conditions in this group are attention-deficit hyperactivity, which is characterized by at least six months of disruptive behaviour before age 7 that impairs a child’s ability to make friends and perform in school; and Tourette’s, which commonly appears before age 21.

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