Reports of qualitative sex differences in the genetic or environmental influences on antisocial behavior are somewhat rare in the extant literature, with only a few twin studies finding evidence of sex-specific genetic or shared environmental effects on antisocial behavior (Derks et al., 2007; Eley et al., 1999; Vierikko et al., 2003; Rose et al., 2004). Part of the reason for the isolated nature of these findings is that relatively few studies have examined qualitative sex differences. Additionally, qualitative sex effects may be small and require large samples, and, in particular, large samples of antisocial females, in order to be detected. Thus, some studies reporting null findings may not have had adequate power to detect this type of sex effect. Of the six studies that reported null findings for qualitative sex differences and reported opposite-sex and same-sex twin (or sibling) correlations (Eaves et al., 1997; Jacobson et al., 2002; Saudino et al., 2005; Slutske et al., 1997; Tuvblad et al., 2005; Van Hulle et al., 2003), five reported twin/sibling correlations that were suggestive of sex-specific etiologic influences. In the present study,
