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Testosterone is significantly correlated with aggression and competitive behaviour and is directly facilitated by the latter. There are two theories on the role of testosterone in aggression and competition.  The first one is the challenge hypothesis which states that testosterone would increase during puberty thus facilitating reproductive and competitive behaviour which would include aggression.  Thus it is the challenge of competition among males of the species that facilitates aggression and violence.  Studies conducted have found direct correlation between testosterone and dominance especially among the most violent criminals in prison who had the highest testosterone levels.  The same research also found fathers (those outside competitive environments) had the lowest testosterone levels compared to other males. 
Similar to Male Extra, L-Arginine HCL is a staple in almost all male enhancement supplement products. L-Arginine is transformed into nitric oxide in the human body, assisting the body to improve both absorption of nutrients and the overall flow of blood. Studies performed have proven the effectiveness of L-Arginine on length and intensity of erections. Concerning male enhancement, L-Arginine is one of the most scientifically advanced ingredients present. Trial results ranged from significant erection improvements to even the doubling of nitric oxide levels. ( Read more here )