Aggressiveness is a major individual difference, or “personality,” factor in most species of animals (Gosling, 2001), including monkeys (Chamove, Eysenck, & Harlow, 1972) and dogs (Svartberg & Forkman, 2002). Strains of mice and rats differ markedly in aggressiveness. In nonhuman animals, aggression is classified by the provoking conditions including: pain, fear, maternal protection of the young, an intruder in the animal's territory, mating competition, and prey to a predator. The last of these, called “predatory aggression,” differs from intraspecies or “defensive aggression” in that it does not require anger or even strong emotion. If anything, the excitement of a predator is more like the positive excitement during sensation seeking in humans. Even well-fed cats seem to enjoy hunting and when there is no real prey they play at stalking and pouncing. Human predators, however, are different in that they stalk their own species, but some of the sensation seeking motivation may be the same as in other species. Sexual sadists are generally unemotional and detached but get pleasurable excitement from the suffering of their victims (Cosyns, 1998). Humans who hunt other species are most like predator animals. Like cats, they hunt for the challenge and pleasure of the kill even when they are not at all hungry.

Human aggression is classified by the form it takes. Human aggressiveness involves three types of expression: the behavioral, as in physical or verbal aggression; the emotional, as in anger; and the attitudinal, as in hostility.