The assumption of random mating for aggression in the parents of the twins is also important to consider, since nonrandom mating can lead to increased resemblance for DZ but not MZ twin pairs. Assortative mating in the parent generation acts to increase the resemblance between dizygotic twins and thereby bias shared environmental estimates upward and additive genetic effects downward. A significant correlation between spouses for a particular trait is often interpreted as assortative mating (Maes et al., 1998). This assumption is probably violated when it comes to antisocial and aggressive behavior, as significant spouse correlations have been found suggesting that assortative mating exists in this behavioral domain (Krueger et al., 1998; Maes et al., 2007; Taylor et al., 2000). Taylor et al., 2000 found that parents of twins were correlated for retrospectively reported delinquency (r = 0.23 in families of boys and r = 0.35 in families of girls) and concluded that assortative mating is modest in degree. Another study using data from the Dunedin sample in New Zealand (Silva and Stanton, 1996) when the participants were 21-years-old found a
