The Society for Neuroscience states that aggression is a "complex social behavior" and can be divided into three categories: predatory aggression, social aggression and defensive aggression. A person with predatory or social aggression seeks out a target, while defensive aggression is a response.

Some drugs, particularly when used to excess and/or used illegally, significantly increase the risk for aggressive behavior and the commission of acts of violence. Alcohol, anabolic steroids, benzodiazepines, and cocaine are key drugs that can escalate the development of aggression and violent behavior.

Infoplease defines aggression as behavior that can be directed toward others or internally, and which can be either constructive or destructive. Humans are certainly not alone in expressing aggression against one another. However, aggression in human beings is often motivated by factors beyond physical survival.