In psychology, as well as other social and behavioral sciences, aggression refers to behavior between members of the same species that is intended to cause pain or harm. Predatory or defensive behavior between members of different species is not normally considered "aggression." Aggression takes a variety of forms among humans and can be physical, mental, or verbal. Aggression should not be confused with assertiveness, although the terms are often used interchangeably among laypeople, e.g. an aggressive salesperson.
There are two broad categories of aggression. These include hostile, affective, or retaliatory aggression and instrumental, predatory, or goal-oriented aggression. Empirical research indicates that there is a critical difference between the two, both psychologically and physiologically. Some research indicates that people with tendencies toward affective aggression have lower IQs than those with tendencies toward predatory aggression. If only considering physical aggression, males tend to be more aggressive than females. One explanation for this difference is that females are physically weaker than men, and so need to resort to other means.