The aim of this study was to determine whether there are sex differences in regard to the effect of early childhood co-socialization on inbreeding-avoidance at sexual maturity. Multiple logistic regression methodology was used to model the relationship between cousin relationship and social proximity in childhood and its effect on mate choice beyond adolescence for a sample of Moroccan participants. Females showed a synergistic interaction effect for cousin marriage and childhood co-socialization and an additional interaction effect for co-socialization during the first seven years of childhood. Male participants showed no significant effect for early childhood co-socialization. The Westermarck Effect hypothesis that early childhood co-socialization leads to a lack of sexual interest at maturity is thus supported for female participants but not for male participants. These results are consistent with expectations from parental investment theory that females should show greater inbreeding-avoidance than males do because of the greater costs females suffer as a consequence of inbreeding depression.