Man is the only animal that practices war and mass murder. The key seems to be in our social learning process. Aggression is everywhere, everyday. Wars, examples of sanctioned aggression, and murders, examples of hostile aggression are the most publicized. However, more subtle forms of aggression stemming from the desire to achieve a goal, or the desire to deny someone else their goal are just as common.
Males show higher levels of physical aggression, while females are more verbally aggressive.
This aggression results from a combination of heredity and environment, with a major emphasis on environmental factors. Gender roles are one reason for this. Boys are encouraged to take part in more aggressive activities than girls are. Role models are also an important factor in aggression. Studies show children imitate the behaviors of their adult models.