How kind or mean we are is related to the size of a part of our brains, say researchers.
Thirty volunteers were asked to have a brain scan while playing a computer game in which they had to split money between themselves and anonymous partners.
Those who made generous decisions were found to have a larger temporoparietal junction, compared with those who made stingy decisions.
The study, published in the journal Neuron, was carried out by researchers at the University of Zurich.
Dr Yosuke Morishima said: "These tendencies don't change in the short term - nice people don't become mean overnight - and this area may explain the variation between individuals."
