C onsiderable evidence from twin and adoption studies indicates that both genetic and shared environmental factors play a substantial role in the liability to antisocial behavior. Although twin and adoption designs can resolve genetic and environmental influences, they do not provide information about assortative mating, parent-offspring transmission, or the contribution of these factors to trait variation. We examined the role of genetic and environmental factors for conduct disorder (CD) using a twin-parent design. This design allows the simultaneous estimation of additive genetic, shared and individual-specific environmental effects, as well as sex differences in the expression of genes and environment in the presence of assortative mating and combined genetic and cultural transmission. A retrospective measure of CD was obtained from twins and their parents or guardians in the Virginia Twin Study of Adolescent Behavior Development and its Young Adult Follow up sample. Both genetic and environmental factors play a significant role in the liability to CD. Major influences on individual differences appeared to be additive genetic (38%-40%) and unique environmental (39%-42%) effects, with smaller contributions from the shared environment (18%-23%), assortative mating (~2%), cultural transmission (~2%) and resulting genotype-environment covariance. This study showed significant heritability, which is slightly increased by assortative mating, and significant effects of primarily nonparental shared environment on CD.In this article, we aim to accomplish four goals. First, we highlight some of the main findings published so far with respect to individual differences in internalizing and externalizing behavior from a range of perspectives. Second, we briefly introduce the genetic epidemiologic literature on antisocial behavior, with a special focus on longitudinal applications. Third, we describe the overall design, sample and measures in the Virginia Twin Study of Adolescent Behavior Development (VTSABD) and Young Adult Follow Up (YAFU) in detail as well as the measures on antisocial behavior used in the current analyses. Fourth, we illustrate some of the unique features of its design by extending the classical twin study with data collected from biological and nonbiological (referred to as stepfor this article) parents to examine the role of genetic and environmental factors in the variation of antisocial behavior. The added information from parent-offspring correlations and spousal correlations will allow us to (i) disentangle genetic and cultural transmission, (ii) partition the environmental variance into influences shared with parents, with twin siblings and those specific to the individual, and (iii) test for and quantify the effects of assortative mating. This design thus provides an extended perspective to the traditional longitudinal study by studying the sources of continuity and change across generations. Although we only illustrate this using retrospective data in adolescence and one single measure, these...