Testosterone is a sex hormone; It is naturally produced in both men and women but is made at a much higher level in men. Low testosterone is not just about libido or becoming stronger and bigger in the gym. While this can be an effect of treatment, the condition known as Low T is much more in-depth. It is what gives a male the secondary sex characteristics of a man. Testosterone – which can profoundly reduce energy levels – is responsible for stimulating muscle growth, achieving normal erectile and cognitive functions, restore energy levels and maintaining normal bone density.
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.