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.     
Active Life: 2-3 days
Drug Class: Anabolic/Androgenic Steroid (Injectable)
Effective Dose: Men 150-450 mg/every 2nd or 3rd day
Water Retention: Yes, high
High Blood Pressure: Yes
Liver Toxic: Low
Aromatization: Yes, high
DHT Conversion: Yes, high
Decrease HPTA function: Yes, severe
Testosterone Propionate is a commonly manufactured, oil-based injectable testosterone compound. The Propionate ester will slow the rate in which the steroid is released from the injection site, but only for a few days. Testosterone Propionate is therefore much faster acting than other testosterone esters such as cypionate or enanthate, and requires a much more frequent dosing schedule, in order to maintain stable blood levels. While cypionate and enanthate are injected on a weekly or bi-weekly basis, propionate is usually injected every second. The propionate ester can be very irritating to the site of injection. In fact, many sensitive individuals choose to stay away from this steroid completely, their body reacting with a pronounced soreness and low-grade fever that may last for a few days.
Those who do not mind frequent injections will find propionate to be quite an effective steroid. As with all testosterones, it is a powerful mass drug, capable of producing rapid gains in size and strength. At the same time the buildup of estrogen and DHT (dihydrotestosterone) will be pronounced, so typical testosterone side effects are to be expected. Many consider propionate to be the mildest testosterone ester, and the preferred form for the dieting/cutting phases of training. Some will go so far as to say that propionate will harden the physique, while giving the user less water and fat retention than one typically expects to see with a testosterone. Realistically however, this is nonsense. The ester is removed before testosterone is active in the body, and likewise the ester cannot alter the activity of the parent steroid in any way, only slow its release. We can say that propionate might be the favored testosterone among female bodybuilders (for those who insist on testosterone use), as blood levels are easier to control with it compared to other esters. Should virilization symptoms develop, one would not wish to wait the weeks needed for testosterone concentrations to fall after a shot of enanthate for example.
During a typical cycle one will see action that is consistent with a testosterone. Those particularly troubled by gynecomastia may find that a combination of Nolvadex works especially well at preventing/halting this occurrence. And as with all testosterone products, propionate will also suppress endogenous testosterone production. The use of a testosterone stimulating drug like HCG and/or Clomid/Nolvadex is therefore a requirement in order to avoid enduring a post-cycle crash.