Several years ago, Ernst Fehr returned from a trip with gifts for his four-year-old son and eight-year-old daughter. Fehr, an economist at the University of Zurich who studies the evolution of human cooperation, had removed the price tags so that the children would not rate one gift more highly than the other. But when they began measuring the boxes with a ruler to see which was larger, and — in their view — better, he wondered at what age children begin to understand the concept of fairness.

“Three-year-olds will use the language of fairness, but mainly to their own advantage and to support their egoism,” says Fehr. Benevolence and sensitivity toward others are critical virtues that allow humans to cooperate with genetically unrelated individuals. But little was known about when young children begin to take account of the welfare of others.

To find out, Fehr and his colleagues enlisted the help of some pint-sized participants from playgroups, kindergartens and primary school classes. They designed three experiments that would measure the children's 'other-regarding' preferences, using Smarties, Jelly Babies and Fizzers as appealing currency.

In each test a child was asked to choose between two actions: the 'equal shares' choice of taking one packet of sweets for themselves and one for their partner, or an alternative that tested aspects of regard for others — 'prosocial' behaviour (actions that benefit another; in which the alternative choice was no sweets for their partner), 'envy' (in which the alternative choice was one packet for the child and two for their partner), and 'sharing' (in which the other choice was two for the child and none for their partner).

They found that young children develop a particular form of other-regarding behaviour, called inequality aversion, between the ages of three and eight. On average, three- and four- year-olds behave selfishly whereas seven- and eight-year-olds prefer situations that remove the inequality (see page 1079). “It's not simple generosity, it is egalitarianism,” says Fehr. “If the other child has less, then I'm willing to give, but I don't want the other child to have more than me either.”

Fehr says he was surprised by how strongly egalitarianism increases with age, particularly in the envy game. Almost 90% of seven- and eight-year-old children prefer to distribute the sweets equally rather than give their partner extra. Also surprising was that 15–20% of the children minimized their partner's payoff in all three games. Such 'spiteful' behaviours are also seen in about 10–15% of adults. “These children could be an interesting group to study. Maybe spiteful adults have spiteful children,” he says.

The researchers made some interesting observations outside of the tests. They sent the parents a questionnaire that asked about the child's siblings, birth order and his or her ability to feel empathy. The package included a payoff of sorts — extra postage stamps, “something that every parent could use”, says Fehr — to encourage parents to return the questionnaire. “Thank you for the stamps,” some of the parents wrote back. “The interesting thing,” says Fehr, “is that the children of those parents who wrote a thank you note were more prosocial.”