As for testosterone; you understand how important testosterone is to the human body; in-fact, it is one of the most important hormones we produce. Through the use of anabolic steroids our natural testosterone production is suppressed and while total suppression varies with each and every steroid, Nandrolone steroids have a 100% suppression rating. Without testosterone supplementation your body will not have the testosterone it needs to function properly; no matter how special you believe you may be or what your friend may have told you, if you supplement with a Nandrolone hormone you wont be making any testosterone. It is important to note when Deca 300 use comes to and end and once it and any other anabolic steroids have cleared your system your natural testosterone production will begin again; however, during the actual cycle it will not exist. By supplementing with exogenous testosterone we eliminate this problem, we provide our body with the testosterone it needs to function and as a bonus only enhance our Deca 300 use as the testosterone hormone in of itself is highly anabolic.
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.