Sometimes nothing can stop you, or set you back, sometimes you are so bad and aggressive ...What's behind these moods that you are in from time to time?

The drive to restart a confrontation after losing a challenge seems to depend on the testosterone levels, as psychologists from University of Texas at Austin have found. The researchers investigated why some fight back after losing a challenge in a contest while others back down after a failure; they discovered a connection to the men's testosterone levels after a competition.

The team studied more than 60 men who competed against each other in pairs, measuring participants' testosterone levels and charged the men to complete the task of tracing through a pattern of numbers. After the confrontation, the researchers measured the men's testosterone levels and asked whether each would like to compete again. Among the losers, 70 % of those whose testosterone levels increased wanted to compete again, while 80 % of those whose levels dropped refused to compete again.

But changes in testosterone levels did not predict who would want to compete again among the winners. Perhaps winners may not be interested in facing again an opponent because the re-match might result in a loss. The researchers also wanted to see the basis for the participants' changes in their testosterone levels. Surprisingly, the result of the competition had no effect on the men's testosterone levels. Rather, pre-competition stress levels in participants - stress measured by cortisol (the stress hormone) - were powerful predictors of post-competition testosterone change, especially among losers.

Previous studies had shown that people with high levels of cortisol also have high chronic levels of stress and anxiety. The stress of losing may depress the release of catecholamines, chemical compounds such as dopamine and noradrenaline, which can cause a drop in testosterone.

It is known that changes in testosterone levels get reflected on human behavior. "The study suggests our social behavior may, in part, be driven by changes in our hormone levels," said Pranjal Mehta, one of the researchers. "When testosterone levels increase, we seem to become more dominant and driven to gain status. But, when testosterone levels drop, we seem to become more submissive."

The researchers told that similar study should be made on women to determine whether the same pattern emerges. There is far less research on testosterone and social behavior in women, but higher levels of testosterone in women may be associated with assertive, aggressive and dominant behaviors.