Scientists say they have found the part of the brain that predicts whether a person will be selfish or an altruist.
The brain area was more active among the altruistic group
Altruism - the tendency to help others without obvious benefit to oneself - appears to be linked to an area called the posterior superior temporal sulcus.
Using brain scans, the US investigators found this region related to a person's real-life unselfish behaviour.
The Duke University Medical Center study on 45 volunteers is published in Nature Neuroscience.
The participants were asked to disclose how often they engaged in different helping behaviours, such as doing charity work, and were also asked to play a computer game designed to measure altruism.
The study authors say their work could have important implications.
They are now exploring ways to study the development of this brain region in early life and believe such information may help determine how altruistic tendencies are established.
Researcher Dr Scott Huettel explained: "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."
Dr George Fieldman, member of the British Psychological Society and principal lecturer in psychology at Buckinghamshire Chilterns University College, said it was conceivable that there would be a region of the brain involved with altruism.
He added: "If you can educate from an early stage to be more altruistic that would be good for the community, and if you could also show that had an impact on brain development that would be very interesting."
He said true altruism was a rare or even intangible thing.
"Altruism is usually reciprocal - you do something for someone and you expect something back ultimately.
"The other types are kin altruism, giving to ones relatives, and being cheated or cuckolded."
He said it would be interesting to study people at the extremes of altruism and selfishness and see if their brains differed significantly.