The University of Oregon researchers also found that the pleasure is more intense when the giving is voluntary.
In the study in the journal Science, 19 women volunteers were given $100 each. Their brains were scanned as they were shown their money automatically being transferred from their account to a local food bank, and again as they were given the opportunity to donate the money. In both cases, areas of the brain associated with pleasure processing were activated.
One of the authors of the paper, Ulrich Mayr, told Reuters: "What is interesting is that these pleasure areas are for really basic needs, like food, sex, sweets, shelter and social connection. It's the area that tells the brain what is good for us."
He said the results showed there is a pleasure in giving over and above the social and egotistical reward of being a philanthropist.
"The fact that we find pleasurable activity in those mandatory tax-like situations strongly suggests the existence of pure altruism," he said.
The Institute of Fundraising's director of policy and campaigns, Megan Pacey, called the report "interesting", but cautioned that the results of the study might not be replicated in the UK, where the tax system provides fewer inducements to philanthropy than its American equivalent.
A spokesman for the donor information website Intelligent Giving also reacted cautiously to the paper. He said: "We're not convinced by the sample size or method. This is anecdotal rather than scientific, but it adds to the body of evidence that giving to charity makes you feel good."