Not all twin studies have examined sex-limited effects, either qualitative or quantitative, and several studies combined males and females when computing twin correlations, making it impossible to evaluate these effects based on published results shown in Table 8.2. Nonetheless, quantitative sex differences can be easily evaluated across at least 18 studies in Table 8.2, which present separate twin or sibling correlations by sex. Among these, there are a dozen studies that also include MF, which allow investigation of qualitative sex differences. The average twin correlations across these 18 studies, weighted by their respective sample sizes, shows quite similar twin correlations for both identical (rMZ–Males= 0.66; rMZ–Females= 0.63) and nonidentical same-sex pairs (rDZ–Males= 0.42; rDZ– Females= 0.35), indicating that there are no appreciable quantitative sex differences in aggressive behavior. This is consistent with the individual results across studies which formally tested 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
