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. 
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Testosterone is a steroid hormone produced in the adrenal glands of both sexes and in the testes of males and the ovaries of females. Testosterone is largely responsible for the formation and maintenance of male sex characteristics, including both the larger bone and muscle development seen in males. The testosterone levels in humans are regulated by hormones released from the brain; in males the hypothalamus and pituitary glands in the brain increase testosterone during puberty and male characteristics develop (for example, penile enlargement, facial hair, interest in sex).