There are different reasons why a person may act aggressively towards other human beings. The person may act this way because of his culture or the way he was brought up in society. The person does not, however, act this way based on instinct alone. Aggression is a molded, learned behavior. A human being must have both environmental and instinctual factors in order to display aggression. Some of a person's natural instincts are to desire food, reject certain things, escape from danger, fight when challenged, sex desire, care for the young, dominate, and to accept inferior status. The combination of instincts and environment determines a person's behavior. This is based on the theory that everything human beings do would have to be learned from other human beings. Aggression must be learned; it is not simply there from birth. Rather than being an uncontrollable instinct, a person's behavior is something that is taught to him. For example, a newborn baby is breathing because it is an involuntary reflex. On the other hand, a father may tell his young son to beat up the school bully who is picking on him. As a result, the boy is taught to deal with the situation by using violence.
In order for an individual to display aggression, it must be driven by an instinct interacting with that person's surroundings. McDougall defines the word instinct as "an inherited or innate psycho-physical disposition which determines its possessor to perceive, and to pay attention to, objects of a certain class, to experience an emotional excitement of a particular quality upon perceiving such an object, and to act in regard to its particular manner, or, at least, to experience an impulse to such action." This definition basically explains that people have different reactions for different stimuli. Therefore, an individual is prone to act a certain way when he is stimulated to do so from his surrounding environment.