There is mixed evidence regarding qualitative and quantitative sex differences in the heritability of conduct disorder. Quantitative sex differences refer to differences in the degree to which genetic influences account for variance in a phenotype in males and females, and qualitative sex differences refer to the degree to which the genetic influences on a phenotype overlap for males and females (Neale et al., 2006). Some have reported no significant quantitative sex differences (Gelhorn et al., 2005); however, others have reported significant qualitative or quantitative sex differences. In a population-based US twin sample, Jacobson et al. (2002) found evidence of quantitative sex differences, such that genetic influences for conduct disorder were more pronounced for females compared to males in childhood but not adolescence. However, they did not find evidence for qualitative sex differences. The absence of qualitative sex differences suggests that the genes that influence conduct disorder in males are the same genes that influence conduct disorder in females. In contrast, in a population-based Australian twin sample, Meier et al. (2011) found that a model that allowed the genetic influences on
