Biology and social science research has studied gift-giving, but the former has been more concerned with courtship and the latter has come from either a cultural-relativistic perspective or a handicap principle perspective. We argue that our understanding of gift-giving in humans can be enhanced by examining animal models as long as the model-species shares the appropriate behavior: monogamy. Thus, the gibbon might be a more appropriate model. Monogamy encourages pairs to expend effort in mate-retention. In Study 1 (N = 120), we show that gift-giving in courtship is localized to long-term mates: most strongly in men. In Study 2 (N = 100), we demonstrate that gift-giving is a tactic used by men to both court and retain mates: most commonly for retention. In line with traditional models of helping, women planned to provide gifts to friends and family more than men. We also demonstrate that sociosexuality predicts planned expenditure on gifts to different individuals and that these correlations are moderated by the sex of the participant.

As the fields of personality and evolutionary psychology merge, new conceptualizations of personality traits will arise. However, individual differences continue to present a recurrent problem for evolutionary psychologists. We contend that the heterogeneity of personality traits provides fitness benefits and fitness costs that relate to fitness outcomes. As these costs and benefits balance one another in the course of individuals' lives, only those traits that create a net positive balance will be correlated with improved fitness outcomes. In this study (N = 155), we showed how extraversion provides positive fitness outcomes to both sexes, suggesting that extraversion creates the best balance of positive and negative characteristics and habits of all of the Big Five. In contrast, other aspects of the Big Five were not related to number of lifetime sex partners, suggesting that the positive and negative aspects of those traits create a near-neutral balance.