I have taken Nugenix for about 3 years. I swear by it. When I was about 60 my testosterone levels had dropped to about 50T before I did anything about it. I was extremely depressed almost incapable of doing my job. I am a professional in an office responsible for about 150 clients.
My first doctor gave me a T gel that I rubbed in. My second doctor began a testosterone injection regimen 200 mg every two weeks. Believe it or not that did return me to active sexual activity. It wasn’t until I started to take a similar product at GNC that put the hard back in an erection. After starting the injections I could get a limp erection but I could not perform. I could do my job and the injections put a swagger back into my step. Nugenix was the break over point for me. That is my experience without using any medical language.
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.     
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