The aim of this research was to investigate the claim that gender differences in levels of social dominance orientation (SDO; Pratto, Sidanius, Stallworth, & Malle, 1994), a personality variable measuring a general predisposition towards anti-egalitarianism, are essentially invariant (Sidanius & Pratto, 1999). Previous findings have indicated that (regardless of covariate) males display higher levels of SDO than females. Two studies were conducted to test the expectation (derived from social identity theory) that the gender-SDO relationship would be moderated by strength of gender group identification. Both samples (150 non-students and 163 students) completed the full SDO(6) measure, and measures of gender group identification. Consistent with predictions, strength of gender identification was found to moderate the gender-SDO relationship, such that increasing group identification was associated with increasing SDO scores for males, and decreasing SDO for females. This result raises questions concerning the theoretical basis of social dominance theory, and whether gender group membership should be accorded a different status from other 'arbitrary-set' group memberships.