The studies summarized in Tables 8.1 and Tables 8.2 vary on how aggressive behavior was defined (i.e., physical, verbal, relational, reactive, proactive, indirect, bullying) and measured (observation, self-report, parent/caregiver, teacher). A wide range of ages were included, from preschool children to adults; however, the vast majority of studies have used childhood samples (i.e., 12 years of age or younger) which explains why the CBCL is so frequently used to assess aggressive behavior. Correlations for biological and adoptive siblings (Table 8.1), and MZ and DZ twins (Table 8.2) are shown for each study. Most studies reported correlations separately for same-sex pairs of males (M), females (F), and opposite-sex pairs (MF); however, a few studies involve correlations for samples of male and female pairs combined. We review the key questions concerning the genetic influence (heritability) of human aggression based on the effect sizes reported for these studies. We also examine various potential moderators of these effects, including sex, age, method of assessment, form of aggression, study design (twin vs. sibling adoption design), and various social factors and circumstances that may exacerbate or ameliorate the genetic risk for aggression from one person to the next.
