As men and women age their testosterone levels diminish. Testosterone is known as the Hormone of Desire . In middle-aged men, low testosterone symptoms (Low T) or Andropause start to appear. Some of the most profound symptoms of low testosterone are - feeling extremely tired all day, unexpected weight gain especially increased belly fat, loss of muscle or muscles getting flabby, low sex drive or a complete lack of desire, softer erections or complete erectile dysfunction, inability to sleep or outright insomnia, memory and hair loss, bursts of anger, social withdrawal and depression.
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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.