Does donating a chunk of change to charity make you feel good inside? How about paying your taxes? According to a new study, people have similar brain activity—and feel rewarded—no matter how they fund the public good.

Researchers at the University of Oregon set out to determine the motives of people who give to charity. Their findings, reported this week in Science: people contribute out of "pure altruism"— the pleasure of seeing the others' well-being improve—as well as for the "warm glow" they feel as the result of doing a good deed.

Their study involved subjects who played a game in which they were forced to fork over money to a fictional soup kitchen, which the experimenters equated with paying "taxes," although in real life taxpayers rarely see their tax dollars go directly to good works.

The forced "taxed" condition was alternated with situations in which the players voluntarily decided whether—and if so how much—to donate to the soup kitchen. In addition, subjects received a wad of "cash'' referred to by researchers as "pay offs,'' which were unrelated to donation decisions.

Scientists used functional magnetic resonance imaging (fMRI) to monitor the players' brain activity during the game.

The scans showed increased activity in the nucleus accumbens, an area of the brain linked to receiving rewards in both humans and animals, when participants received "money" in the game—a well-established finding. There was also increased activity when people voluntarily gave away "money", which indicated that they felt a "warm glow" because of their decision to help others. But scientists were surprised to discover that there was also heightened activity in the same brain region when they were forced to give to charity under the "taxed" condition.

"It supports one idea of why people give: because of the common good," says study co-author Ulrich Mayr, a cognitive psychologist. The similar brain activities, however, dispute most economic theories, which are based on the notion that people almost always act out of self-interest and not out of concern for the welfare of others.

In fact, there was more brain activity in some subjects when they were forced to give than when they received the "payoffs.'' The researchers labeled these subjects as "altruists" and compared them with "egoists," whose brains were activated more when they received the monetary rewards.

According to study co-author William Harbaugh, an economist, the researchers could predict whether a player would voluntarily pony-up funds to the soup kitchen based on their brain activation pattern.

"That's the definite innovation of the paper: to predict behavior based on [brain] activation," says Brian Knutson, a professor at Stanford University who researches economic theory using brain imaging but was not involved in this study.

Researchers say the finding that people have a sense of reward even under "taxed" conditions may be useful for evaluating tax policies. But the researchers acknowledge there are differences between paying blind taxes (that indirectly fund services) as opposed to those from which they see immediate benefits. "We show that people liked paying a tax that went to a food bank," Harbaugh says. "But suppose the tax had been unfair. What then?"

Lest you think this is evidence of the goodness of mankind, take note: when subjects donated instead of being taxed, they gave 33 percent less than those forced to give to the soup kitchen.