Although genetic studies of aggression have spanned from childhood to adulthood, most studies included in Tables 8.1 and Tables 8.2 involved children 12 years of age or younger. This suggests that more studies examining the heritability of aggressive behavior in adolescents and adults are needed. Keeping this in mind, it is useful to examine the magnitude of twin correlations across age groups, which span from early childhood to middle-age adults. These correlations are summarized in Fig. 8.1, according to five age groups (early childhood, age 1.5–6 years; middle childhood, age 7–10; adolescence 11–15; late adolescence/young adulthood, age 16–26; and adulthood, age 27–48; Fig. 8.1). These results show that aggressive behavior is clearly influenced by genetic factors across the lifespan, given the fact that the MZ correlations exceed those for DZ pairs at all ages. (The lack of qualitative sex differences 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
