Twin temperament during the transition from infancy to early childhood.

A multiple-regression model for the analysis of twin data in which one twin's score is predicted from that of its co-twin, the coefficient of relationship, and the interaction term provides direct estimates of heritability (h2) and the proportion of variance due to shared environmental influences (c2). Several multiple-regression models were fitted to parental ratings of infant and toddler temperament for 306 pairs of twins. High estimates of h2 were found for eight of the nine temperament scales, and three scales showed significant effects of c2. No differential heritability was found between males and females across the infant and toddler years.