Experimental programming of life histories: Toward an experimental science of individual differences

Four experimental variables were combined in a 2 × 2 × 2 × 2 factorial design. These were (1) infantile handling vs. no handling of the mothers of the subjects, (2) infantile handling vs. no handling of the subjects, (3)rearing of the subjects in either a maternity cage or a free environment between birth and weaning, and (4) rearing of the subjects in either a laboratory cage or a free environment between weaning and 42 days of age (N = 6 Purdue-Wistar rats per group). Starting at 220 days of age, the groups were given a battery of tests which measured emotional reactivity, exploratory behavior, and consumption-elimination. Analysis of the criterion data revealed the following: (1) handling pups in infancy reduced emotional reactivity and this reduction was found to be invariant with respect to different combinations of life history experiences; (2) exploratory behavior was markedly influenced by the animal's pattern of life experiences; (3) when mothers were handled during their infancy, their offspring explored significantly less than offspring of nonhandled mothers.