This article examines the empirical evidence for a genetic influence in the etiology of antisocial behavior. This review relates the results of studies from 3 approaches to genetic investigation. The first, family studies, provides valuable information about the increased risk for deviance among the family members of affected individuals. Family studies provide few conclusions about genetic etiology, however, because members of families share environments as well as genes. A second approach, the study of twins, offers a somewhat better separation of genetic and environmental effects. The twin studies compare monozygotic (MZ) twins, who are genetically identical, to fraternal, same-sex, dizygotic (DZ) twins who have no more genes in common than other siblings (50%). The research design assumes that the effect of hereditary factors is demonstrated if the MZ twins have more similar outcomes (concordance for deviance) than DZ twins. The twins are reared together in almost all studies, and the environmental influences for MZ pairs may be more similar than for DZ pairs. A third approach, the adoption study, largely overcomes the possibility of confounding genetic and environmental factors which limit inferences from the results of twin studies. In this method, the deviant outcomes of adopted children (separated early in life from their biological parents) are compared with the outcomes of their adoptive parents and their biological parents. Similarity in outcome between adoptees and biological parents indicates a genetic effect.