The Classification of Intellectual Disability

Although intellectual disability has been recognized since antiquity, interest in its classification did not develop until the nineteenth century, when it became apparent that intellectual disability is not one homogeneous category, as was previously thought, but has many causes. Moreover, it became apparent that intervention could be beneficial and that interventions might be tailored for specific disorders. Early authors prepared the way for modern efforts to differentiate specific conditions that differ in both etiology and pathology, yet all result in intellectual disability. Some attempts were misguided. J. Langdon Hayden Down, in his ethnic classification (1866; Jordan, 2000), sought to classify based on the physical appearance of the individuals he examined. His goal was to absolve parents of self-blame for the handicap by emphasizing a constitutional basis for their child’s disorder. He proposed an “ethnic classification,” suggesting that the various forms of intellectual disability represented regressions to stereotypical racial forms (e.g., mongoloid, Aztec). Although he later abandoned this unfortunate idea, he continues to be known for it. Still, he is credited with drawing scientific attention to the syndrome bearing his name (Jordan, 2000) and for suggesting that the best classification is one based on etiology. Subsequently, he anticipated current efforts at classification by describing three major groups: (1) congenital, which included microcephalic, macrocephalic, hydrocephalic, epileptic, and paralytic types; (2) developmental, with a vulnerability to mental breakdown with stress during a developmental crisis; and (3) accidental (caused by injury or illness). Later, William Weatherspoon Ireland (1877), in his textbook on intellectual disability, suggested 10 subdivisions. Among these are genetous (congential), microcephalic, epileptic, eclamptic, hydrocephalic, paralytic, traumatic, inflammatory, cretinism, and idiocy by social and physical deprivation. In 1880, tuberous sclerosis complex was identified by Désiré-Maglione Bourneville (1880), who established that intellectual disability might result from brain pathology. Subsequently, many other intellectual disability syndromes were recognized. Thus began a new era, with investigators searching for clearly defined disorders associated with intellectual disability; these were commonly named after their discoverers. It was an era when intellectual disability syndromes were beginning to be recognized, but medicine had little to offer therapeutically.