Why can castration reduce aggression in some animal species?

In many vertebrate species, castration reduces aggression because it removes the primary source of testosterone, an androgen produced by the testes.12

Brain structures involved in controlling aggression are often richly supplied with receptors that bind to these steroid hormones. In a wide range of species, there is a clear correlation between a male’s circulating testosterone levels and his level of aggressiveness; consequently, castration typically leads to a dramatic decrease in aggressive behavior. This effect is reversible, as experimental reinstatement of testosterone can restore aggression.1

However, the relationship is complex. The impact of castration is less pronounced in species with more elaborate social structures, and aggression can still be modulated by testosterone produced by the adrenal gland or by other hormones, such as vasopressin. Additionally, while castration affects adult behavior, early developmental exposure to hormones also plays a permanent role in organizing brain structures to be more or less sensitive to aggression-facilitating signals later in life.13