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Previous research has linked the facial width-to-height ratio (FWHR) to a host of psychological and behavioral characteristics, primarily in men. In two studies, we examined novel links between FWHR and sex drive. In Study 1, a sample of 145 undergraduate students revealed that FWHR positively predicted sex drive. There were no significant FWHR × sex interactions, suggesting that FWHR is linked to sexuality among both men and women. Study 2 replicated and extended these findings in a sample of 314 students collected from a different Canadian city, which again demonstrated links between the FWHR and sex drive (also in both men and women), as well as sociosexuality and intended infidelity (men only). Internal meta-analytic results confirm the link between FWHR and sex drive among both men and women. These results suggest that FWHR may be an important morphological index of human sexuality.

Correlational research suggests that men show greater attraction to feminine female faces when their testosterone (T) levels are high. Men's preferences for feminine faces also seem to vary as a function of relationship context (short versus long-term).However, the relationship between T and preferences for female facial femininity has yet to be tested experimentally. This thesis examined the causal role of T in modulating preferences for facial femininity across both short and long-term mating contexts, using two separate experiments. Results of Experiment 1 (within-subject design, n = 24)showed that participants significantly preferred feminized versus masculinized versions of women's faces. Further, participants showed a stronger preference for feminine female faces in the short-versus the long-term context after they received T, but not after they received placebo. Post-hoc analyses suggest that this effect was driven by a lower preference for feminine faces in the long-term context when on T relative to placebo.Results from Experiment 2 (between-subject design, n = 93) were highly consistent with those of Experiment 1: men demonstrated a significant preference for feminized female faces in the short-versus the long-term context after T, but not after placebo administration, and this effect was driven by lower preferences for feminine faces in the long-term context when on T relative to placebo. Collectively, these findings provide the first causal evidence that T modulates men's preferences for facial femininity as a function of mating context.

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Testosterone (T) concentrations change rapidly in the context of human competition, and these changes in neuroendocrine function may serve to modulate future aggressive behavior. However, an increase in T during competition does not translate into aggressive behavior among all individuals. Here, we examined the extent to which individual differences in trait anxiety moderate the relationship between T responses to competition and aggressive behavior. Across two studies, we found that T responses to competition were positively correlated with subsequent aggression, but only among men scoring relatively low in trait anxiety. Trait anxiety did not moderate the relationship between T reactivity and aggression in women. These findings highlight the importance of considering individual difference in trait anxiety when examining the neuroendocrine correlates of human aggression.

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One of the most widely studied biological correlates of aggressive behavior is the steroid hormone testosterone. Although traditional wisdom might suggest that individuals with more testosterone are more likely to be aggressive, research over the past several decades has identified important contextual, individual difference, and methodological variables that are key moderators of any such effect. In this chapter, we review literature examining how aggression is linked with baseline levels of testosterone, how testosterone fluctuates rapidly within the context of human competitive behavior, and how such competition‐induced hormonal fluctuations serve to potentiate ongoing and/or future aggressive behavior. The neuroendocrine mechanisms underlying such complex social behavior are discussed from research conducted within humans as well as nonhuman species, providing comparative clues as to the adaptive nature of such intricate systems.