This seminal authority in animal behavior and originator of many concepts such as 'critical periods' in animal learning and socialization here provides a thorough study of aggressive behavior. The author views aggression as arising from multiple causes, such as the level of organization in the population of animals exhibiting such behavior. Social animals may ritualize fighting to reduce group stress and individual injury, whereas individual predators may learn how to fight as an extension from observation of adults seeking mates, defending territory or taking prey. This thorough examination of aggression is thought-provoking, especially in its relevance to human behavior.