Along with prescription boosters, there are also a number of herbal and vitamin based supplements that are marketed as natural options to increase testosterone levels. A natural testosterone booster is often a combination of vitamins and minerals that are considered necessary for the efficient function of male and female reproductive systems as well as supporting the development of various gender based traits. In most cases, these products are not regulated by any type of national medical association. While the products may not create much in the way of side effects, most rely on anecdotal rather than proven scientific evidence to advance their claims.
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.