Within a sample of 584 twins aged 12 to 25 months (292 pairs) studied longitudinally, positive affect measured through two laboratory pleasure episodes and maternal report at 12 and 22 months significantly predicted empathy-related helping and hypothesis testing assessed between 19 and 25 months. Girls showed significantly more concern than did boys, whereas boys engaged in hypothesis testing significantly more than did girls. Behavior-genetic analyses indicated substantial shared environmental influences for positive affect and empathy composites of helping and concern. Genetics and the nonshared environment influenced empathy-related hypothesis testing. The best fitting bivariate model included shared and nonshared environmental influences on positive affect and helping, with environment accounting for the covariation between the two traits. The covariation between positive affect and hypothesis testing was genetically influenced. (Contains 2 figures and 6 tables.)