Scientists have found that testosterone undermines the way women trust other individuals, protecting them from exploitation, but only if they're more socially naive in the first place.
Writing in PNAS, Utrecht University researcher Peter Bos and his colleagues show that 24 young female volunteers given a small dose of testosterone rated 150 photographed faces as significantly less trustworthy than they did in the absence of the hormone. Interestingly, the effect was also most manifest amongst the women awarding the highest "trustworthiness" ratings to the photographs to begin with; their more cautious counterparts, on the other hand, were much less affected by the agent.
This suggests that testosterone might function as the hormonal opposite of oxytocin, which has been shown to trigger sensations of trust towards other individuals and is also thought to play a role in mother-baby and partner-partner bonding. In this way, testosterone can boost social vigilance to prevent a person being taken advantage of. It has also been shown to promote rational decision-making and shrewdness, as evidenced by previous research revealing that stockbrokers with higher morning testosterone levels make more money.
The downside of too much of the stuff, however, is that it makes individuals over-aggressive, over-competitive and over-suspicious, leading to a loss of social benefits; this is probably why the entire population isn't permanently "T'd" up to the max...