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The extended twin kinship design allows the simultaneous testing of additive and nonadditive genetic, shared and individual-specific environmental factors, as well as sex differences in the expression of genes and environment in the presence of assortative mating and combined genetic and cultural transmission (Eaves et al., 1999). It also handles the contribution of these sources of variance to the (co)variation of multiple phenotypes. Keller et al. (2008) extended this comprehensive model for family resemblance to allow or a flexible specification of assortment and vertical transmission. As such, it provides a general framework which can easily be reduced to fit subsets of data such as twin-parent data, children-of-twins data, etc. A flexible Mx specification of this model that allows handling of these various designs is presented in detail and applied to data from the Virginia 30,000. Data on height, body mass index, smoking status, church attendance, and political affiliation were obtained from twins and their families. Results indicate that biases in the estimation of variance components depend both on the types of relative available for analysis, and on the underlying genetic and environmental architecture of the phenotype of interest. [ABSTRACT FROM AUTHOR]

Considerable 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 (approximately 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. [ABSTRACT FROM AUTHOR]

Illicit substance use is dangerous in both acute and chronic forms, frequently resulting in lethal poisoning, addiction, and other negative consequences. Similar to research in other psychiatric conditions, whose ultimate goal is to enable effective prevention and treatment, studies in substance use are focused on factors elevating the risk for the disorder. The rapid growth of the substance use problem despite the effort invested in fighting it, however, suggests the need in changing the research approach. Instead of attempting to identify risk factors, whose neutralization is often infeasible if not impossible, it may be more promising to systematically reverse the perspective to the factors enhancing the aspect of liability to disorder that shares the same dimension but is opposite to risk, that is, resistance to substance use. Resistance factors, which enable the majority of the population to remain unaffected despite the ubiquity of psychoactive substances, may be more amenable to translation. While the resistance aspect of liability is symmetric to risk, the resistance approach requires substantial changes in sampling (high-resistance rather than high-risk) and using quantitative indices of liability. This article provides an overview and a practical approach to research in resistance to substance use/addiction, currently implemented in a NIH-funded project. The project benefits from unique opportunities afforded by the data originating from two longitudinal twin studies, the Virginia Twin Study of Adolescent and Behavioral Development and the Minnesota Twin Family Study. The methodology described is also applicable to other psychiatric disorders.

Background: Longitudinal, genetically informed, prospective data collected on a large population of male twins (n=1037) were used to examine developmental differences in the etiology of antisocial behavior., Method: Analyses were carried out on both mother- and child-reported symptoms of conduct disorder (CD) in 10- to 17-year-old twins from the Virginia Twin Study of Adolescent Behavioral Development (VTSABD) and self-reported antisocial behavior by the twins as young adults from the Young Adult Follow-Up (YAFU) study., Results: The following trends were identified: (1) a single genetic factor influencing antisocial behavior beginning at age 10 through young adulthood ('life-course persistent'); (2) a shared-environmental effect beginning in adolescence ('adolescent-onset'); (3) a transient genetic effect at puberty; and (4) a genetic influence specific to adult antisocial behavior., Conclusions: Overall, these etiological findings are consistent with predictions from Moffitt's developmental theory of antisocial behavior. The genetic effect at puberty at ages 12-15 is also consistent with a genetically mediated influence on the timing of puberty affecting the expression of genetic differences in antisocial outcomes.