subjects took part in a behavioral experiment where the distribution of a real amount of money was decided. The rules allowed both fair and unfair offers. The negotiating partner could subsequently accept or decline the offer. The fairer the offer, the less probable a refusal by the negotiating partner. If no agreement was reached, neither party earned anything. Test subjects with an artificially enhanced testosterone level generally made better, fairer offers than those who received placebos, thus reducing the risk of a rejection of their offer to a minimum. Two later studies have empirically confirmed these results. However men with high testosterone were significantly 27% less generous in an ultimatum game. The Annual NY Academy of Sciences has also found anabolic steroid use (which increases testosterone) to be higher in teenagers, and this was associated with increased violence. Studies have also found administered testosterone to increase verbal aggression and anger in some participants. A few studies indicate that the testosterone derivative estradiol (one form of estrogen) might play an important role in male aggression. Estradiol is known to correlate with aggression in male mice. Moreover, the conversion of testosterone to estradiol regulates male aggression in sparrows during breeding season. Rats who were given anabolic steroids that increase testosterone were also more physically aggressive to provocation as a result of "threat sensitivity". The relationship between testosterone and aggression may also function indirectly, as it has been proposed that testosterone does not amplify tendencies towards aggression but rather amplifies whatever tendencies will allow an individual to maintain social status when challenged. In most animals, aggression is the means of maintaining social status. However, humans have multiple ways of obtaining social status. This could explain why some studies find a link between testosterone and pro-social behaviour if pro-social behaviour is rewarded with social status. Thus the link between testosterone and aggression and violence is due to these being rewarded with social status. The relationship may also be one of a "permissive effect" whereby testosterone does elevate aggression levels but only in the sense of allowing average aggression levels to be maintained; chemically or physically castrating the individual will reduce aggression levels (though it will not eliminate them) but the individual only needs a small-level of pre-castration testosterone to have aggression levels to return to normal, which they will remain at even if additional testosterone is added. Testosterone may also simply exaggerate or amplify