Although there is a great deal of interest in the Behavioral Inhibition System (BIS) and Behavioral Activation System (BAS)-two temperaments formulated by Gray's reinforcement sensitivity theory-few studies have examined the genetic and environmental etiology underlying their continuity and change. Using a two-wave longitudinal data set of twins from late adolescence to early adulthood, this study examined (1) whether genetic influences contribute to change as well as continuity of the two temperaments and (2) whether the magnitude of genetic influences on the BIS and BAS differs across the two measurement points. The questionnaire was administered to 448 twin pairs at two waves, with an interval of approximately 2-3 years. Univariate genetic analyses revealed that genetic factors accounted for around one-third of the phenotypic variance in the BIS and BAS traits at both waves. Longitudinal bivariate analyses revealed that (1) genetic influences contribute only to continuity, whereas environmental influences contribute to both continuity and change in the two traits, and (2) the degree of genetic influences does not differ across time. These results suggest that in this age period, temporal stability of individual differences in the BIS and BAS owes more to genetic than to environmental factors.
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