Sex Differences in Direct Aggression:
The Role of Impulsivity, Target Sex, and Intimacy with the Target

Abstract

Men are, as a sex, more aggressive than women. Evolutionary accounts of the sex difference in direct aggression focus on the differing costs and benefits of aggression for men and women, and posit that male aggression and female non-aggression are both part of a suite of adaptations to sex-specific selection pressures. However, greater male aggression is not evident in studies of intimate partner aggression conducted in Western cultures. The present thesis sought to integrate evolutionary accounts of the sex difference in direct aggression with research on intimate partner aggression showing gender symmetry in aggressive acts.
The proposed proximate mechanisms for the sex difference in direct aggression are myriad, but one of the most extensively investigated is impulsivity. The present thesis therefore sought to establish the presence or absence of sex differences in impulsivity, and identify the forms of impulsivity most likely to mediate the sex difference in aggression. Chapter Two presents a meta-analysis of sex differences in psychometric and behavioural measures of impulsivity. Sex differences are consistently present on those forms of impulsivity which are affective or motivational as opposed to cognitive in nature, and which incorporate some element of risk. Risky impulsivity, a personality trait reflecting a tendency to take risks without prior thought, was identified as a strong candidate for mediating the sex difference in aggression.
In Chapter Three, the role of risky impulsivity in same-sex aggression and sociosexuality, both of which are related to the pursuit of reproductive success in the face of risk, was examined. Results from this chapter indicate that risky impulsivity might represent a common proximate mechanism for individual differences in aggression and sociosexuality, but that explaining sex differences in direct aggression requires consideration of processes at the dyadic, as well as the intrapsychic, level.
Finally, the reasons for the absence of a sex difference in intimate partner aggression were examined more closely in Chapter Four. Participants were asked about hypothetical responses to provocation by same-sex friends, opposite-sex friends, and partners. Self-report data were also gathered on participants' actual aggressive behaviour towards partners, same-sex friends and strangers, and opposite-sex friends and strangers. There was good concordance between vignette responses and self-reports. Results indicated that men's aggression is inhibited towards all female targets relative to male ones, but that women's aggression is disinhibited specifically towards partners. In other words, men's lowered aggression towards intimate partners is an effect of target sex, while women's raised aggression towards intimate partners is an effect of intimacy with the target.
It is argued that gender parity in intimate aggression is the result of sex-specific influences on rates of perpetration. It is further argued that any complete account of sex differences in aggression must be able to account for gender symmetry in aggression towards intimate partners. To this end, due consideration should be given to sex differences in low-level emotional and motivational processes, particularly fear, as well as the effects of sex differences in styles of anger expression. Specifically, men's reduction in intimate partner aggression might be best explained by the effects of Western social norms which proscribe aggression towards all women, while women's raised intimate partner aggression might be best explained by an oxytocin-mediated reduction in fear which is specific to intimate partners.