In our study, it was difficult to resolve whether the qualitative sex difference observed for CD was genetic or shared environmental in origin. It should be noted that twin designs are generally not well-suited to resolving the source of a qualitative sex difference. Nonetheless, our finding of a smaller correlation among opposite-sex compared to same-sex twin pairs for CD is clear evidence of factors (whether they are genetic or environmental or both) that are having an impact on one sex but not the other. If sex-specific genetic effects on CD exist, there are several possible mechanisms by which these sex-specific genetic risk factors could emerge (Prescott, Aggen, & Kendler, 1999). For example, sex differences in genetic background, gene-environment interaction, or gene-environment correlation could account for a finding of a qualitative genetic sex difference. An alternative, but not mutually exclusive, explanation is that antisocial behavior is associated with X-linked genes. There is some evidence from molecular genetic studies that antisocial behavior is associated with X-linked genes. A particular variant of the MAOA gene, an X-linked gene, appears to confer risk for antisocial behavior, at least in the presence of adverse environmental circumstances (Caspi et al., 2002; Kim-Cohen et al., 2006).
