It is every parent's observation that children begin to show 'social' sharing behaviour only when they reach the age of seven or eight, however hard you try to program them earlier.

Professor Ernst Fehr of the University of Zurich showed children of different ages a photo of another child and asked them to distribute portions of jelly beans and other small sweets.

Nearly 80% of the older children preferred to divide the available sweets evenly rather than unequally, and 40% of them - in a variation of the game - chose to give some sweets to the other child even when they could have kept all of them. In a variation the researchers labelled 'envy', however, the children usually preferred to go without extra sweets if it meant giving still more to the other child.

Only 9% of younger children were prepared to treat the other child fairly.

"I think that both genes and culture play a role," said Professor Fehr. "Nobody would dispute that the sexual maturation of children is driven by biology and genes, so why should other phenotypes, like those associated with fairness behaviour, not also be driven by biology and genes?" He thinks however that the results suggest that; "social norms of equality can come into being even without extended forms of cultural transmission."

In 2006 Professor Fehr reported research which located control of fairness behaviour in a part of the frontal lobes known as the dorsolateral prefrontal cortex, or DLPFC. Previous researchers had suggested that the DLPFC suppresses our judgement of fairness, but Fehr's research showed the opposite: that the region suppresses our natural tendency to act in our own self interest.

He used transcranial magnetic stimulation to temporarily shut off activity in the DLPFC, with the result that, in circumstances when people would normally reject a low cash offer in a transaction game, subjects were actually more likely to take the money.

"The DLPFC is really causal in this decision. Its activity is crucial for overriding self interest," said Fehr. When the region is not working, people still know the offer is unfair, he says, but they do not act to punish the unfairness.

The DLPFC is far better developed in humans than in any other species, although it does not reach its full capability until the early twenties. Again, every parent knows that this is the age at which those Masters of the Universe we call teenagers start to show some small signs of social maturity.

It is hard to resist the conclusion that the expansion of the DLPFC (and many other brain adaptations) was part of the evolutionary process that made it possible for humans to live in socially complex groups. But the cultural programming processes we call parenting and education are also important.