is also evident across the life span, in that the DZ correlation is comparable for same-sex and MF pairs across ages.) However, both MZ and DZ correlations decline steadily across development, suggesting the waning importance of shared environmental effects from childhood to adolescence and then adulthood. The DZ correlation exceeds half the value of the MZ correlation (taken as evidence for shared environment) only in early childhood, but not in later age groups. The pattern shown in Fig. 8.1 is evident in individual studies as well. Aggressive behavior in childhood is influenced by genetic factors in all studies, and most of these studies also report shared environmental influences (Table 8.2; e.g., Baker et al., 2008; Eley et al., 1999; Hudziak et al., 2003; Schmitz et al., 1995; Simonoff et al., 1998; Tuvblad et al., 2009; van den Oord et al., 1996; Vierikko et al., 2004; but for an exception see Dionne et al., 2003; Taylor, 2004). Studies including adolescent twins (∼ younger than 19 years of age) do not in most cases report finding any shared environmental influences (Table 8.2; e.g., Button et al., 2004; Cho et al., 2006; Gelhorn et al., 2006; Tackett et al., 2009). Similarly, studies including
