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.     
Clinical research still hasn’t determined a hard threshold level for when symptoms of low T begin appearing . Some recent research suggests that symptoms of low T might begin appearing in men when their total testosterone level dips below 320 ng/dl . According to anecdotal evidence from the owner of Peak Testosterone, many men start noticing low T symptoms when their total testosterone dips into the 400s . Of course, it’s anecdotal, so take it for what it’s worth, but it’s probably a good idea to stay above 500 ng/dl if you don’t want to experience symptoms of low T.