Despite the acceptance of evolutionary theory in the biological and social sciences, typological thinking—the belief that individual differences diverge around an underlying type or essence—has persisted. The most egregious example is J. H. Langdon Down's “ethnological classification of idiocy,” taken seriously in the field for almost 80 years after its origin in 1866. Past and present controversies over the definition of mental retardation have turned on an unacknowledged typological axis. Some contemporary research on Down syndrome indicates that the allure of typology is still an obstacle to the appreciation of individual differences and human dignity in the field of mental retardation.

An earlier version of this paper was presented at the meeting of the Cheiron Society, New Brunswick, Maine, June 1995. The author thanks Ann Donnellan for bringing the work of Burton Shapiro to his attention.

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