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Testosterone

Early androgenization has an organizational effect on the developing brains of both males and females, making more neural circuits that control sexual behavior as well as intermale and interfemale aggression become more sensitive to testosterone. There are noticeable sex differences in aggression. Testosterone is present to a lesser extent in females, who may be more sensitive to its effects. Animal studies have also indicated a link between incidents of aggression and the individual level of circulating testosterone. However, results in relation to primates, particularly humans, are less clear cut and are at best only suggestive of a positive association in some contexts.

In humans, there is a seasonal variation in aggression associated with changes in testosterone. For example, in some primate species, such as rhesus monkeys and baboons, females are more likely to engage in fights around the time of ovulation as well as right before menstruation. If the results were the same in humans as they are in rhesus monkeys and baboons, then the increase in aggressive behaviors during ovulation is explained by the decline in estrogen levels. This makes normal testosterone levels more effective. Castrated mice and rats exhibit lower levels of aggression. Males castrated as neonates exhibit low levels of aggression even when given testosterone throughout their development.