Abstract
Estimates of genetic and environmental influences on externalizing behavior are markedly inconsistent. In an attempt to refine and extend our knowledge of externalizing behavior, the current study examined the etiology of externalizing behavior using observational data in middle childhood and adolescence from three twin and sibling samples. Observational ratings offer a unique perspective on externalizing behavior rarely examined within behavioral genetic designs. Shared environmental influences were significant and moderate to large in magnitude across all three samples (i.e., 44, 77, and 38%), while genetic influences (31%) were significant only for the adolescent sample. All three samples showed greater shared environmental influences and less genetic influence than is typically found when examining self-, parent-, and teacher-reports of externalizing behavior. These findings are consistent with other reports that have found evidence for shared environmental influences on measures of child externalizing behavior-in direct contrast to a commonly held perception that shared environmental factors do not have significant influences on behavior beyond early childhood.