Men commit most violent crimes. Scientists have studied the role of testosterone, the male sex hormone, in the violent and aggressive behaviour.

In the laboratory, male mice castration reduces aggression while testosterone replacement restores aggression in castrated mice. The male hormone has an effect on the organisation of the brain. The brain in infancy is sensitive to the effects of the male hormone, which make it more responsive to the effect of testosterone in later life.

However, male individuals differ in their aggressiveness. In one experiment, scientists separated male mice into two groups: the aggressive and the non-aggressive. They were then castrated which reduced their aggression. When they gave testosterone replacement therapy, only those mice that belonged to the aggressive group showed a restoration of aggressive behaviour. In other words, testosterone is necessary for aggressive mice to exhibit aggression, but replacing testosterone is not sufficient to turn a previously non-aggressive mouse into an aggressive mouse. A possible explanation for this finding is the idea that early exposure to testosterone organizes the brain. Testosterone also affects females. In female mice, which had their ovaries removed, testosterone - but not estrogen - increases aggressiveness.

Studies about the relationship between hormones and aggression in humans use various methods such as interviews, questionnaires or criminal records to assess aggressive behaviour. It does appear that there is a positive correlation between violence in male and female prisoners and circulating levels of androgen. Nevertheless, it is unclear whether androgens facilitate aggression or just encourage social dominance, competitiveness, and impulsiveness.

It is possible that high levels of testosterone secreted inside the body encourage behaviour, which aims to dominate and enhance one's status over other people. Sometimes dominant behavior is aggressive when its clear intent is to inflict harm on another person, but often dominance is aggressive. Sometimes dominant behavior takes the form of antisocial behavior, which includes rebellion against authority and law breaking.

The act of competing for dominant status affects male testosterone levels in two ways. The first, testosterone rises in the face of a challenge, as if it were a response to an impending competition. The second, testosterone rises in winners and declines in losers after the competition. Thus, there is a two ways relationship between testosterone and dominance behavior, each one affect the other.

There are important individual differences in our response to testosterone. In a recent study, research gave testosterone to 56 men aged 20 to 50 years. Testosterone treatment significantly increased aggressive responses on a frustration-inducing computer game involving a fictitious subject. This effect, however, was not uniform across individuals; most showed little psychological change, whereas a few developed prominent effect.

In sports, testosterone increases before competition as a response in anticipation to winning. At the same time, winning increases testosterone while losing reduces testosterone level. These effects also occur in sports fans. The same effects happen in non-physical games such as chess. It is possible that these effects are due to changes in status because testosterone increases in students after graduation and in hostages after their release from captivity. Thus, it is possible that these effects will correlate to changes in dominance. Studies have shown that testosterone levels change in response to changes in ranking of men.

There are two possibilities, either testosterone causes changes in person' dominance or dominance affects the level of testosterone. If testosterone level is a stable personal trait, it is possible to predict a person's aggressive, dominant and antisocial behaviour from measuring this hormone in his blood.

Men with higher levels of testosterone are more likely to be arrested for offenses (other than traffic violations), to buy and sell stolen property, incur bad debts, or use a weapon in fights.

On the other hand, testosterone levels vary according to a person's dominance. In an interesting study of 2,100 male Air Force veterans, testosterone levels fell and remained low with marriage and rose with divorce. It seems that marriage make men lose their dominance.