Aggressiveness is an ancestral behavior common to all animal species. Its neurophysiological mechanisms
are similar in all vertebrates. Males are generally more aggressive than females. In this review,
aggressive behavior in rodents, monkeys, and man and the role of testosterone and brain
serotonin levels have been considered. Interspecifi c aggressiveness in rats has been studied considering
the mouse-killing behavior; the neonatal androgenization of females increases adult mousekilling
as does the administration of testosterone in adults. Intraspecifi c aggressiveness was studied
by putting two or more male rats (or mice) in the same cage; the condition of subjection or dominance
is infl uenced by testosterone.
In monkeys, testosterone is related to aggressiveness and dominance and, during the mating season,
increases in testosterone levels and aggressive attitude are observed. In men, higher testosterone
levels were obtained in perpetrators of violent crimes, in men from the army with antisocial
behaviors, in subjects with impulsive behaviors, alcoholics and suicidals, in athletes using steroids,
and during competitions. Aggressive and dominant behavior are distinguished. Testosterone infl
uences both of these, even if man is usually inclined to affi rm his power without causing physical
damage. Testosterone receptors are mainly in some hypothalamic neurons, where it is aromatized
into estrogens, which determine the increase in aggressiveness. A relation between testosterone
levels and diencephalic serotonin has been shown: in fact, the lack of serotonin increases aggressive
behaviors both in animals and man. Testosterone also increases ADH levels in the medial amygdala,
lateral hypothalamus, and preoptical medial area, involved in aggressive behaviors