(AP) Bill Clinton said he felt others' pain. But a new brain-scanning study suggests that when guys see a cheater get a mild electric shock, they don't feel his pain much at all. In fact, they rather enjoy it.

In contrast, women's brains showed they do empathize with the cheater's pain and don't get a kick out it.

It's not clear whether this difference in schadenfreude _ enjoyment of another's misfortune _ results from basic biology or sex roles learned during life, researchers say. But it could help explain why men have historically taken charge of punishing criminals and others who violate societal rules, said researcher Dr. Klaas Stephan.

Stephan, a senior research fellow at the University College London, is co-author of a study led by Tania Singer at the college and published online Wednesday by the journal Nature.

Singer, in an e-mail message, said the sex difference in results was a surprise and must be confirmed by larger studies. The researchers said women might have reacted like men if the cheater suffered psychological or financial pain instead.

The scientists scanned the brains of 16 men and 16 women after the volunteers played a game with what they thought were other volunteers, but who in fact were actors. The actors either played the game fairly or obviously cheated.

During the brain scans, each volunteer watched as the hands of a "fair" player and a cheater received a mild electrical shock. When it came to the fair-player, both men's and women's brains showed activation in pain-related areas, indicating that they empathized with that player's pain.

But for the cheater, while the women's brains still showed a response, men's brains showed virtually no specific reaction. Also, in another brain area associated with feelings of reward, men's brains showed a greater average response to the cheater's shock than to the fair player's shock, while women's brains did not.

A questionnaire revealed that the men expressed a stronger desire than women did for revenge against the cheater. The more a man said he wanted revenge, the higher his jump in the brain's reward area when the cheater got a shock. No such correlation showed up in women.

Philip Jackson, who studies brain systems responsible for empathy at the University of Laval in Quebec City in Canada, said he found the sex differences intriguing and worth following up on.

The overall results elegantly tie together "a lot of things we either knew or suspected strongly" about how social interaction can affect the brain's activity, he said.

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