This review of family, twin, and adoption studies on offender behaviors points to the importance of a genetic predisposition as a partial explanation for criminality in some populations. Twin and adoption studies show that criminality increases with the presence of criminality in biological relatives; the environmental effects of having a criminal adoptive parent also increase the risk of adoptee criminality. Delinquency is described developmentally as a fairly typical phase for many adolescents. Because of the high base rate for delinquency, any genetic predisposition for offense behaviors may be difficult to uncover in adolescence. Three groups are differentiated that may aid in this detection: continuous antisocials are delinquents who continue to be criminal as adults; transitory delinquents are delinquents but not criminals; and late bloomers are criminals but not delinquents.