Origins of variation in behavioral style: a longitudinal study of temperament in young twins.

Psychologists' ratings of infant and childhood behavioral style, or temperament, made during the course of the nationwide Collaborative Perinatal Project supplied the raw data for the largest longitudinal twin study yet reported on this topic. The behavioral ratings are available for ages 8 months, 4 years, and 7 years on approximately 350 twin pairs. These ratings were factor analyzed, and co-twin similarity for identical and fraternal twins was examined at each age. The most salient results were as follows: (1) At 8 months, individual differences in a broadly based "activity" factor showed evidence of moderate genetic influences; (2) at 4 years, IQ showed appreciably greater familial influence than any of the composites of temperament ratings, but significant genetic effects were apparent for "task persistence" and "irritability" factors; and (3) some evidence for genetic effects on "active adjustment" and "'fearfulness" factors was present at 7 years. A general conclusion for all ages is that nonfamilial sources account for at least one-half of the observed variance. No strong evidence for sex differences emerged from the analyses.