Testosterone, an essential precursor of estrogen in women, is made in the ovaries and adrenal glands. There is a steady decline in testosterone levels from the 20s through menopause. With surgical menopause, the level of testosterone drops precipitously. No clear lower limit of testosterone has been established; however 15 ng per dL ( nmol per L) commonly is used. One study 38 found that women with 0 to 10 ng per dL (0 to nmol per L) had markedly decreased sexual desire in all situations and absent or markedly decreased orgasms. Because of studies like this, supplemented with anecdotal evidence, many women have been started on testosterone therapy.
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.