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Correlation and causality. Why is it that throughout the animal kingdom and in every human culture, males account for most aggression and violence? Well, what about testosterone and some related hormones, collectively called androgens, a term that unless otherwise noted, I will use simplistically as synonymous with testosterone. In nearly all species, males have more circulating testosterone than do females, who secrete small amounts of androgens from the adrenal glands. Moreover, male aggression is most prevalent when testosterone levels are highest; adolescence and during mating season in seasonal breeders. Thus, testosterone and aggression are linked. Furthermore, there are particularly high levels of testosterone receptors in the amygdala, in the way station by which it projects to the rest of the brain, the bed nucleus of the stria terminalis, and in its major targets, the hypothalamus, the central gray of the mid-brain, and the frontal cortex. But these are merely correlative data. Showing that testosterone causes aggression requires a subtraction plus a replacement experiment. Subtraction, castrate a male: do levels of aggression decrease? Yes, including in humans. This shows that something coming from the testes causes aggression. Is it testosterone? Replacement: give that castrated individual replacement testosterone. Do pre-castration levels of aggression return? Yes, including in humans, thus testosterone causes aggression. Time to see how wrong that is. The first hint of a complication comes after castration. When average levels of aggression plummet in every species, but crucially, not to zero, well, maybe the castration wasn't perfect, you missed some bits of testes, or maybe enough of the minor adrenal androgens are secreted to maintain the aggression. But no, even when testosterone and androgens are completely eliminated, some aggression remains, thus some male aggression is testosterone independent. This point is driven home by castration of some sexual offenders, a legal procedure in a few states. This is accomplished with chemical castration, administration of drugs that either inhibit testosterone production or block testosterone receptors. Castration decreases sexual urges in the subset of sex offenders with intense, obsessive, and pathological urges. But otherwise, castration doesn't decrease recidivism rates as stated in one meta-analysis. Hostile rapists and those who commit sex crimes motivated by power or anger are not amenable to treatment with the anti-androgenic drugs. This leads to a hugely informative point. The more experience the male had being aggressive prior to castration, the more aggression continues afterward. In otherwise, the less his being aggressive in the future requires testosterone and the more it's a function of social learning.
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Robert M. Sapolsky (Behave: The Biology of Humans at Our Best and Worst)