Understanding and Evaluating Emotional and Behavioral Impairment

In English property law, intellectual disability and mental illness were differentiated in the thirteenth century. By 1690, John Locke’s An Essay Concerning Human Understanding had clarified differences between intellectual disability and mental illness. Locke wrote: . . . The defect in [intellectual disability] seems to proceed from want of quickness, activity, and motion in the intellectual faculties, whereby they are deprived of reason; whereas mad men seem to suffer by the other extreme. For they do not appear to me to have lost the faculty of reasoning: but having joined together some ideas very wrongly . . . they argue right from wrong principles. . . . But there are degrees of madness as folly; the disorderly jumbling of ideas together is in some more, and some less. In short, herein seems to lie the difference between [intellectually disabled] and mad men, that mad men put wrong ideas together, and so make wrong propositions, but argue and reason right from them: but [those with intellectual disability] make very few or no propositions, but argue and reason scarce at all. (Scheerenberger, 1983, p. 41) . . . Thus, Locke is often credited with establishing the dichotomy between mental illness and intellectual disability that influenced social policy for people with intellectual disability. But he did not appreciate the capacity persons with intellectual disability do have to reason with adequate supports, nor did he consider that persons with intellectual disability are also at risk for mental illness and behavior disorders. Historically, intellectual disability and mental illness were regarded to be mutually exclusive conditions. Affective and behavioral disturbances in individuals with intellectual disability generally were regarded as manifestations of maladaptive learning and adverse psychosocial experiences rather than as indications of a psychiatric disorder. This view has been shared by both intellectual disability and mental health professionals. Health professionals typically fail to consider the diagnosis of a psychiatric disorder among persons with intellectual disability despite the presence of signs and symptoms that would be readily ascribed as psychiatric disturbance among typically developing persons within the general population. This diagnostic bias may be an outgrowth of several factors.