the overt aggression factor in the current study showed clear contributions of genetic influences but minor or negligible contributions of shared environment. By contrast, prior research points to lesser genetic and more pronounced shared environmental contributions to nonaggressive forms of CD.9,12 The current study clarifies this picture by indicating a more prominent role for genetic influences in rule-breaking symptoms of CD and a more prominent role for shared environmental influences in covert delinquent tendencies. The implication is that studies that combine the 2 into a single nonaggressive symptom category will tend to find weaker genetic-etiologic effects than for aggressive symptoms and stronger shared environmental effects.12
