Sigmund Freud postulated (1920) that all humans possessed an aggressive drive from birth, which, together with the sexual drive, contributed to personality development, and found expression in behavior. Austrian ethologist Konrad Lorenz suggested that aggression was innate, an inherited fighting instinct, as significant in humans as it was in other animals. He contended that the suppression of aggressive instincts, common among human societies, allows these instincts the chance to build up, occasionally to the point where they are released during instances of explosive violence. Many psychoanalysts have argued against these theories, which see aggression as a primary drive, offering the possibility that aggression may be a reaction to frustration of primary needs. In the late 1930s, John Dollard argued that any sort of frustration inevitably led to an aggressive response.

More recently, Albert Bandura has performed studies that indicated that aggression is a learned behavior. Using children in his studies, Bandura demonstrated that, by watching another person act aggressively and obtain desirable rewards or by learning through personal experience that such behavior yields rewards, aggression can be learned. Leonard Berkowitz has contended that all animals learn the most effective response to an aversive occurence (one where the expected reward is denied), whether it be attack or flight. A number of psychologists contend that children and adolescents are vulnerable to media portrayals of violence, particularly in film and television. Popular media tends to depict violence as relatively common, and generally effective. Anonymity may facilitate aggression: when an individual is part of a large group, he may be more likely to elicit aggressive behavior, in a process known as deindividuation.

Recent research on the biological basis of aggression has sought to show that genetic factors may be responsible for aggressive behavior. In the 1970s it was suggested that men who were born with an extra Y chromosome were likely to display more episodes of aggressive behavior than men who were not born with this extra chromosome. Still, conclusive proof has yet to be found for a genetic theory of aggression.

Other factors, including learning difficulties, minimal brain damage, brain abnormalities—such as temporal lobe epilepsy—and such social factors as crowding and poverty have been suggested to have contributed in certain cases to exaggeratedly aggressive behavior. Psychological investigation into aggressive behavior continues, with significant corrolary studies being performed in endocrinology—to determine whether hormonal imbalances have an impact on behavior—and in primate research. Each theory may be accurate in part, since aggression is believed to have a number of determining factors.