Humans engage in aggression when they seek to cause harm or pain to another person. Aggression takes two forms depending on one’s motives: hostile or instrumental. Hostile aggression is motivated by feelings of anger with intent to cause pain; a fight in a bar with a stranger is an example of hostile aggression. In contrast, instrumental aggression is motivated by achieving a goal and does not necessarily involve intent to cause pain; a contract killer who murders for hire displays instrumental aggression. Now, there are many different theories as to why aggression exists in the first place.

Some researchers argue that aggression serves an evolutionary function. Men are more likely than women to show aggression. From the perspective of evolutionary psychology, human male aggression, like that in nonhuman primates, likely serves to display dominance over other males, both to protect a mate and to perpetuate the male’s genes. Sexual jealousy is part of male aggression; males endeavor to make sure their mates are not copulating with other males, thus ensuring their own paternity of the female’s offspring. Although aggression provides an obvious evolutionary advantage for men, women also engage in aggression. Women typically display instrumental forms of aggression, with their aggression serving as a means to an end. For example, women may express their aggression covertly, for example, by communication that impairs the social standing of another person.