Gender is a factor that plays a role in both human and animal aggression. Males are historically believed to be generally more physically aggressive than females from an early age, and men commit the vast majority of murders (Buss 2005). This is one of the most robust and reliable behavioral sex differences, and it has been found across many different age groups and cultures. However, some empirical studies have found the discrepancy in male and female aggression to be more pronounced in childhood and the gender difference in adults to be modest when studied in an experimental context. Still, there is evidence that males are quicker to aggression (Frey et al. 2003) and more likely than females to express their aggression physically. When considering indirect forms of non-violent aggression, such as relational aggression and social rejection, some scientists argue that females can be quite aggressive, although female aggression is rarely expressed physically. An exception is intimate partner violence that occurs among couples who are engaged, married, or in some other form of intimate relationship.