for quantitative sex differences (e.g., Baker et al., 2008; Czajkowski et al., 2008; Eley et al., 1999; Finkel and McGue, 1997; Tackett et al., 2009; Tuvblad et al., 2009). As indicated in Table 8.2, only a small handful of studies have reported significant differences in heritability of aggression for males and females (and these are primarily for younger samples; e.g., Hudziak et al., 2003; van Beijsterveldt et al., 2003; Vierikko et al., 2004). The lack of quantitative sex differences is also well in line with what was reported in a recent meta-analysis summarizing 19 twin and family/adoption studies, whereby genetic influences were found to explain 54% of the variance in aggressive behavior in boys and 57% of the variance in girls (Burt, 2009).
