Hormones and the development and expression of aggressive behavior

Aggression may be defined as overt behavior with the intention of inflicting harm or the threat of harm upon another individual. Aggressive behaviors range from lethal to subtle, and typically arise whenever the interests of two or more individuals conflict. Species- and situation-specific rules exist to regulate aggression. Males, particularly males engaged in territorial or courtship activities, are more aggressive than females in most situations and in most species. The relationship between the testes and aggression has been known since antiquity and castration helps maintain docility among nonbreeding livestock. The long-held relationship between testosterone and aggression has some surprising twists, including the importance of estrogens and their receptors in the regulation of aggression. This chapter outlines what is known about the interactions among aggression and sex steroid hormones, peptides, hormone receptors, neurochemistry, and brain structures in mammals. The ontogeny of aggressive behavior and how hormones and experience shape the trajectory of adult sex differences in aggression is also examined here. Throughout the chapter, a comparison is made between the regulation of rodent animal models of aggression and human aggression with the goal of emphasizing where further research is needed to understand nonadaptive or pathological aggression.