Aggression can be defined as intentionally harming, or causing damage to another person or object. People are usually more aggressive when they are placed in stressful situations. Aggression takes many forms, although it is most obvious when a person physically lashes out at another person. Verbal aggression involves shouting or screaming insults, an example of physical aggression could be hitting someone. Aggression is part of most societies, but is more prominent in some. Aggression can be explained by a biological, or innate theory.

Anger and aggression are usually paired together. Anger is a normal human emotion, and it is how people normally respond to threats, and it inspires powerful, often aggressive feelings that allow people to fight when they are under attack. When anger gets out of control, it becomes destructive and people respond aggressively. People can behave aggressively in order to get what they want (to be rewarded); to be obedient to others; or in reaction to stress such as crowding or noise in their physical environment.

People's perception of aggression is subjective. In some cultures, a raised voice may be interpreted only as a means of getting attention, whereas in others it might be seen as an aggressive act. People's tolerance for aggression may be related to moral or religious beliefs. Part of biological theory states that aggression is a response to the frustration of some goal-directed behaviour. These goals include basic needs like food, water, sleep, sex, love, and recognition.

In the 1930s, John Dollard et al claimed that aggression must always result from frustration and that frustration always leads to aggression. In the 1960s, Leonard Berkowitz modified this theory to say that frustration makes a person ready to be aggressive but does not always lead to aggression. Frustration must be accompanied by anger in order for the person to become aggressive. This anger can be caused by something other than the frustrating situation. This theory has been discredited as there have been other theories that have much more evidence.