Testosterone is a hormone that is produced primarily in the testicles for men and the ovaries and adrenal glands for women. This hormone is essential to the development of male growth and masculine characteristics. For women, testosterone comes in much smaller amounts. Testosterone production increases about 30 times more during adolescence and early adulthood. After early adulthood, it’s natural for levels to drop slightly each year. Your body may see a one percent decline after you’re 30 years old.
Testosterone plays a key role in your:
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.