When Hot Heads Collide: Resident-by-Intruder-Genotype and Genotypically-Mediated Sex Differences in Violent Aggression
While it is widely believed that males commit almost all violent crimes, studies show that women account for over a quarter of arrests for violence. Nevertheless, there are sex differences in the targets of aggression that are thought to have an evolutionary basis in reproductive fitness considerations, with male aggression focused on competition with other males for the limited number of available females, and female aggression focused on competition with other females for high-ranking males who can provide resources to increase the probability of their infants’ survival. The short (s) allele of the serotonin transporter genotype (5-HTT) is implicated in impulsive, violent aggression, but few studies have investigated its differential effects in the sexes. Rhesus monkeys are well suited to investigate sex differences in 5-HTT genotypic effects because they exhibit relatively high base rates of aggression and are genetically similar to humans. The subjects were (N=201 , 79 males, 122 females) genotyped juvenile, adolescent, and adult monkeys, exposed to an unfamiliar age-and-sex-matched intruder while in their home cage. The frequency and target of aggressive behavior of the subjects were recorded. Consistent with predictions from evolutionary-based, sex-specific reproductive goals, males engaged in a higher frequency of violent aggression against the male intruders. Females, on the other hand, were more likely to direct violent aggression toward familiar, same-sex group members. The s-allele amplified these natural tendencies in both sexes, where the frequency of violent aggression increased ten-fold when male residents and intruders both possessed the s-allele. Likewise, female s-allele residents demonstrated three-fold increases in violent aggression toward familiar females. To the extent that the results generalize to humans, sex differences in aggression are in part due to evolutionarily-based, sex-dependent influences that are partially modulated by 5-HTT variation.