What does testosterone do?

Far from always increasing aggression and greed, [testosterone] can actually encourage decency and fair play, scientists have discovered.

The common belief that it makes people quarrelsome, however, can cause it to have that effect. When people think they have been given supplements of the hormone they tend to act more aggressively, even though it does nothing biological to promote such behaviour.

The findings, from an Anglo-Swiss team, suggest that rather than encouraging selfishness and risk-taking as a matter of course, testosterone has subtler effects on human behaviour that depend very much on social circumstances.

- Excerpted from ‘Testosterone’s aggressive impact is a myth. It makes you friendlier' in the Times Online

Scientific American had a related article, noting that Testosterone bumps up status-seeking behavior, not aggressive risk-taking.