Programming Life Histories : Creating Individual Differences By The Experimental Control Of Early Experiences

A homogeneous group of rats was exposed to 16 different patterns of experience in early life. These patterns consisted of the following four variables, combined in a 2 X 2 X 2 X 2 factorial design: (a) type of mother, (b) presence or absence of handling in infancy, (c) type of rearing environment preweaning, and (d) postweaning rearing environment. The 16 groups resulting there from may be conceived of as 16 different experimentally created individuals in which the variance among individuals is a function of known experiential inputs during early life. The nature of the design is such that any genetic contribution to individual differences is of a negligible amount. Twenty-three behavioral tests were administered to the 16 groups. Mean scores for each group on each test were intercorrelated and factor analyzed. Six factors were extracted, three of which could be clearly identified as "emotional reactivity," "consumption-elimination," and "field exploration." The results establish that stable and relatively permanent complex individual differences, of the sort often assumed to be genetically determined, can be generated by the appropriate manipulation of experiences during early life.