The brain responses were measured by a functional M.R.I. machine as a series of transactions occurred. Sometimes the student had to choose whether to donate some of her cash to a local food bank. Sometimes a tax was levied that sent her money to the food bank without her approval. Sometimes she received extra money, and sometimes the food bank received money without any of it coming from her.

Sure enough, when the typical student chose to donate to the food bank, she was rewarded with that warm glow: increased activity in the same ancient areas of the brain -- the caudate, nucleus accumbens and insula -- that respond when you eat a sweet dessert or receive money. But these pleasure centers were also activated, albeit not as much, when she was forced to pay a tax to the food bank.

The most intriguing results were the ones from two of the experimental subjects, students whose brain scans made them definite egoists yet who were also among the most generous in donating. You could dismiss them as statistical outliers, but I like to think we have finally spotted the creature dismissed by so many scholars as myth.

These two women enjoyed no neural reward from charity ? their brains didn?t get enough of a warm glow to compensate for the pain of parting with their money ? yet they made anonymous donations anyway. Diogenes, we may not have found an honest man, but we do seem to have located a couple of true altruists. Either that or two determined masochists.

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