as antisocial behavior and alcoholism. Many studies have also been done on the relationship between more general aggressive behavior and feelings and testosterone. About half the studies have found a relationship and about half no relationship. Studies have also found that testosterone facilitates aggression by modulating vasopressin receptors in the hypothalamus. 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 behavior which would include aggression. It is therefore 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. 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. The rise in testosterone levels during competition predicted aggression in males but not in females. Subjects who interacted with hand guns and an experimental game showed rise in testosterone and aggression. Natural selection might have evolved males to be more sensitive to competitive and status challenge situations and that the interacting roles of testosterone are the essential ingredient for aggressive behaviour in these situations. Testosterone mediates attraction to cruel and violent cues in men by promoting extended viewing of violent stimuli. Testosterone-specific structural brain characteristic can predict aggressive behaviour in individuals. Testosterone might encourage fair behavior. For one study,