Aggression is a complex social behaviour with multiple causes. In psychology, as well as other social and behavioural sciences, aggression refers to behaviour between members of the same species that is intended to cause pain or harm. Aggression takes a variety of forms among humans and can be physical, mental, or verbal. Aggression should not be confused with assertiveness however, although the terms are often used interchangeably. 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. This new book gathers the latest research from around the world in this field.