But what seems to start out well can turn bad, with elevated testosterone levels over several days possibly leading to irrational risk-taking, according to researchers at the University of Cambridge in England.

"If people want to get practical, it would be good for both banks and the financial system as a whole if we had more women and older men in the markets," said John M. Coates, lead author of a study appearing in this week's Proceedings of the National Academy of Sciences.

Such a change would produce a much more stable financial system, said Coates, a research fellow in the university's department of physiology, development and neuroscience.

Coates and Joe Herbert studied male financial traders in London, taking saliva samples in the morning and evening. They found that levels of two hormones, testosterone and cortisol, affected traders.

Those with higher levels of testosterone in the morning were more likely to make an unusually big profit that day, the researchers found.

Testosterone, best known as the male sex hormone, affects aggression, confidence and risk-taking.

Cortisol is tied to uncertainty, novelty and unpredictability, "which pretty much describes a trader's life," Coates said in a telephone interview.

Coates and Herbert's study comes less than two weeks after U.S. researchers reported that young men shown erotic pictures were more likely to make a larger financial gamble than if they were shown a picture of something scary, such as a snake, or something neutral, such as a stapler.

Money and women trigger the same brain area in men, those researchers said.

Camelia Kuhnen, an assistant professor at the Kellogg School of Finance at Northwestern University, said Coates and Herbert's findings "are very interesting, and they help support the claim that emotion influences financial decisions." But she cautioned that the findings don't prove a causal link between testosterone and profitability. Kuhnen, who was not part of Coates and Herbert's team, termed the idea that long-term high testosterone levels can lead to irrational risk-taking an "interesting hypothesis."

The London study indicated hormone levels in the traders were both responding to financial events and influencing them.

Their conclusion: "Cortisol is likely, therefore, to rise in a market crash and, by increasing risk aversion, to exaggerate the market's downward movement. Testosterone, on the other hand, is likely to rise in a bubble and, by increasing risk- taking, to exaggerate the market's upward movement."

And that, Coates and Herbert wrote, "may help explain why people caught in bubbles and crashes often find it difficult to make rational choices."

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