Aggression in male animals and men is very often linked to the sex hormone testosterone. Thus, a decrease in testosterone levels should be followed by a decrease in aggressiveness, and an increase in its levels — by extremely high aggressiveness. Of course, aggression is linked to testosterone, but it cannot be reduced exclusively to the action of a single hormone in isolation from all other neurophysiology. Otherwise, one might arrive at certain misconceptions, for example, that if a high level of testosterone in some men is natural, then violent behavior is also absolutely natural. Additionally, such a view contradicts the concept of the violence inhibitor, according to which many species and even humans possess innate restraints on intraspecific aggression.
To begin, let us recall that the violence inhibitor is linked to the serotonergic system, and the activation of certain serotonin receptors (1A and 1B) can suppress attacking aggression without affecting defensive aggression and other forms of behavior. But how can testosterone affect this effect? As one study shows, there is simply no such influence. Testosterone-driven activation of aggression and serotonin-driven inhibition of aggression work independently of each other; that is, the pro-aggressive effect of testosterone does not consist of suppressing serotonin function. And the influence of serotonin on testosterone-induced aggression is apparently carried out via a parallel inhibitory pathway. It is presumed that this influence occurs in brain regions such as the medial amygdala, hypothalamus, prefrontal cortex, and the septum (septal area), where both sex hormone receptors and serotonergic nerve endings are simultaneously located.
Such an influence of serotonin is necessary to restrain aggression so that it does not cease to be adaptive and functional behavior. This aligns with the violence inhibitor theory, according to which certain internal restraints on aggression prevent it from escalating into violence that causes harm to members of one’s own species. And since testosterone does not disrupt the operation of these restraints, it cannot be the cause of uninhibited aggression. Of course, testosterone increases aggressiveness, but normally only within natural inhibiting limits.
