Similarity in IQ of Separated Identical Twins as Related to Length of Time Spent in Same Environment

Abstract

Hebb (11, pp. 298-299) reported that home reared pet rats, from an ordinary strain of laboratory rats, were significantly superior on a learning task when compared with rats from the same strain reared in a normal laboratory environment. A large number of very well controlled experiments have extended Hebb's initial findings. In a typical experiment of this sort, an experimental group of rats is reared under conditions of enrichment, while matched littermates are reared in the usual laboratory environment. In some cases a third group of rats is reared in an even more deprived environment than the usual laboratory situation. In a representative study of this sort by Forgus (7), infant rats were exposed to visual forms from the time their eyes opened (i6 days) till they reached 41 days of age. A second group of rats was exposed to these same forms during the interval from 41 to 66 days of age. In each case, control populations of littermates were reared without this visual experience. Following the exposure, each group of enriched rats and their controls was tested on a visual discrimination problem. Both enriched groups learned more rapidly than their controls and the early enrichment group learned more rapidly than the late enrichment group. A number of studies (3, 5, 6, 7, 9, 13) of this general variety show substantial agreement. Early deprivation retards later learning while early enrichment increases rate of learning. It has been suggested, e.g., by Hunt (12), McCandless (14, PP. 261-262), Smith and Stone (i9, pp. 6-7), and Thompson (22, p. 33) that studies involving early environmental manipulation of lower organisms have implications with regard to problems of human development. It may be, for example, that enrichment or deprivation of problem solving experience to which a human is exposed prior to ages 2 or 3 is related to later ability. This view finds some support in the position taken by such writers as Spitz (20, 21) and Bowlby (i) regarding the effects of early institutionalization. The Spitz-Bowlby point of view has been subjected to criticism, both in terms of Spitz's experimental procedures (17) and in terms of empirical research on the lasting effects of deprivation (4). Other, earlier, enrichment studies, such as the series of kindergarten enrichment studies conducted by Wellman and her associates (23) have also been subjected to severe criticism (Io). Any further light that can be shed on the effects of environ-