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Little is known about the neurobiological pathways through which testosterone promotes aggression or about the people in whom this effect is observed. Using a psychopharmacogenetic approach, we found that testosterone increases aggression in men ( N = 308) with select personality profiles and that these effects are further enhanced among those with fewer cytosine-adenine-guanine (CAG) repeats in exon 1 of the androgen receptor (AR) gene, a polymorphism associated with increased AR efficiency. Testosterone’s effects were rapid (~30 min after administration) and mediated, in part, by subjective reward associated with aggression. Testosterone thus appears to promote human aggression through an AR-related mechanism and to have stronger effects in men with the select personality profiles because it more strongly upregulates the subjective pleasure they derive from aggression. Given other evidence that testosterone regulates reward through dopaminergic pathways, and that the sensitivity of such pathways is enhanced among individuals with the personality profiles we identified, our findings may also implicate dopaminergic processes in testosterone’s heterogeneous effects on aggression.

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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.