3: Mutability of Intelligence and Epidemiology of Mild Mental Retardation

Abstract

Individuals and groups vary in measured intellectual performance at different times, and here we attempt to order knowledge about this mutability, particularly as it may bear on the distribution of mental retardation. Our object was to find a secure basis for statements about the mutability of intelligence and to set those statements out as testable propositions. Such propositions can find application in practice if they are valid; they can be rejected and supplanted by others if they are invalid; and they can be elaborated and specified if they are too general. There is no need here to enter into controversy on the heritability of intelligence (Conway, 1958; Burt, 1961; Jensen, 1969). Current genetic models emphasize that the relative contribution of heritable and environmental factors is by no means fixed and may change with changed environmental conditions (Edwards, 1969). Identical twins show a high degree of concordance for many characteristics, but for the epidemiologist it is often less fruitful to examine the circumstances in which they resemble each other than the circumstances in which they are dissimilar. The heritable components of a characteristic that has a continuous distribution, like IQ, are assumed to result from multiple genes that are polymorphic; each polymorphic gene may express itself in different forms, each form in different degrees. The multiple forms of expression allow for subtle and complex interaction with the environment. Current genetics, in common epidemiology, uses models of multiple causality. Even where the heritable component of a particular characteristic appears to be large, the models are compatible with dramatic changes in the frequency of the characteristic produced by the environment. Tuberculosis and rheumatic fever, diseases for which a genetic element has seemed established, have dwindled to a small fraction of the rates that existed a half-century ago. Similarly, the heritable component for height and weight has been estimated to be larger than the heritable component of IQ scores, yet this century has seen considerable increases in the height and weight of the populations of industrial societies.