We used the classical twin design to separate the variance in conduct symptoms during early adolescence and use of tobacco, alcohol, and cannabis in young adulthood into that due to additive genetic (A), shared (family) environmental (C), and residual (E) influences (see[26–27]). A represents the variance resulting from the sum of allelic effects across all segregating genes. C refers to environmental influences that have the effect of making twins more similar to one another, such as shared home environment, parenting style, and residential area. The E term includes environmental influences that have the effect of making twins different from one another, stochastic biological effects, as well as measurement error. These variance components can be estimated because MZ twins share all their genetic variation, while DZ twins share on average 50% of their segregating genetic variation. If the variance in a phenotype were completely due to genetic variance, we would expect a twin pair correlation of 1.0 for MZ twins and 0.5 for DZ twins. Both MZ and DZ twin pairs share the C influences, so if C were the only
