The second theory is similar and is known as "evolutionary neuroandrogenic (ENA) theory of male aggression".   Testosterone and other androgens have evolved to masculinize a brain in order to be competitive even to the point of risking harm to the person and others. By doing so, individuals with masculinized brains as a result of pre-natal and adult life testosterone and androgens enhance their resource acquiring abilities in order to survive, attract and copulate with mates as much as possible.  The masculinization of the brain is not just mediated by testosterone levels at the adult stage, but also testosterone exposure in the womb as a fetus. Higher pre-natal testosterone indicated by a low digit ratio as well as adult testosterone levels increased risk of fouls or aggression among male players in a soccer game.  Studies have also found higher pre-natal testosterone or lower digit ratio to be correlated with higher aggression in males.     
If your levels are low, you may be prescribed testosterone replacement therapy. Most men with low testosterone rub a testosterone gel on their arms or shoulders, according to the Urology Care Foundation . Another method is to get a shot into a muscle, or you can wear a patch that slowly releases testosterone into your blood. There are also pellets that go under the skin. There are oral replacement therapies as well, but these are not recommended for testosterone replacement. Men with prostate cancer shouldn’t take testosterone because it can fuel cancer growth.