Genetic and environmental influences on depressive symptomatology in adolescence: individual differences and extreme scores.

The purpose of the current investigation was to compare the genetic and environmental influences on individual differences in depressive symptomatology (as assessed using the Children's Depression Inventory) to such influences on extreme scores in an unselected sample of adolescents. The sample included 707 pairs of siblings (average ages 14.5 and 12.9 years, respectively) participating in a combined twin- and step-family study. Moderate genetic influence was found for the full range of individual differences in depression; in contrast, there was nonsignificant genetic influence, and significant shared environmental influence, on extreme scores. The results were interpreted using a risk model in which familial influences specific to the high end of the distribution contribute to depressive symptomatology in adolescence.