Altruism, one of the most difficult human behaviours to define, can be detected in brain scans, US researchers reported yesterday.
They found activity in a specific area of the brain could predict altruistic behaviour - and people's own reports of how selfish or giving they are.
"Although understanding the function of this brain region may not necessarily identify what drives people like Mother Theresa, it may give clues to the origins of important social behaviours like altruism," said Scott Huettel, a neuroscientist at Duke University in North Carolina who led the study.
They set up an experiment in which they put 45 college students into a functional magnetic resonance imaging scanner, which can take real-time images of brain activity.
They gave the students various games to play, and told them that winning earned cash for either themselves or for a charity. The students had chosen the charities beforehand from a list, the researchers report in the journal Nature Neuroscience.
The students reacted differently depending on whether they won for themselves or for charity with the ones who described themselves as altruistic responding more strongly.
"The game involved reacting as fast as one can to the appearance of a target; if one responds fast enough, then money was earned," Mr Huettel said.
The task was fairly simple, and the students did not give up any payments to themselves to give to the charities. But it cost enough effort that Mr Huettel believes it did represent altruistic intent.
"It's challenging and requires them to focus," Mr Huettel said in a telephone interview. "They are lying in a tube, and it is a little tiring for them. Even though it doesn't cost them anything monetarily, it costs them their effort."
And the researchers were surprised by their findings. Some other studies had predicted that giving would activate the reward systems in the brain.
In fact, another centre was activated when the students either won money for charity, or watched the computer win money for charity.
"This area we saw was the posterior superior temporal cortex," Mr Huettel said. "It's part of the parietal lobe. What this brain area seems to be involved in is extracting meaning from things you see."
"If you see a rock move because someone picked it up, you can recognize that they have a goal. That would activate this region. If you saw a leaf fluttering in the wind, there is no intention in that leaf." And this brain region would not activate.
"We think altruism might help others understand the intentions of others," Mr Huettel said.
His team asked the students how altruistic they were, and found the test strongly correlated with their own reports of unselfish activity, such as helping a stranger or comforting a friend.
He admits it is very difficult to measure altruism.
"If done in the laboratory, it would be difficult to know whether subjects were trying to impress the experimenter with their actions, and thus one could not be sure of the validity of any measurements," Mr Huettel said.