But those with siblings are more selfish than those without.
Children become more egalitarian as they get older, learning the principles of equality by the age of eight, according to a study by Swiss scientists. That may not be too surprising to anyone who has kids - but the researchers also found that having older siblings actually tends to make children more selfish, and that children with no siblings are the least selfish.
Humans are endowed with a sense of fairness that most other animals seem not to share, but it's not been clear exactly when this concept starts to develop.
To find out, Ernst Fehr of the University of Zurich and his colleagues devised a series of tests to measure just how much children care about equality at different ages. The researchers needed to make sure that what they were testing was truly a bias towards egalitarianism, and not just behaviour that seems to be sociable but that could be selfishly driven - for example, giving someone a sweet if you expect to get a sweet in return later.
In three different versions of a game, children were asked to choose between two ways of sharing a number of sweets between themselves and an anonymous partner. They could choose, for example, between 'one for me and one for you', or just having one for themselves. These games tested a variety of attitudes, including a child's preference for giving away sweets at no cost to themselves, and for sharing two sweets rather than keeping both for themselves.
Stop being so selfish
At the age of three, children were "almost completely selfish", says Fehr. They declined to give sweets away even if it made no difference to their own hoard. But by the age of eight, children generally preferred the egalitarian option, splitting a prize equally rather than keeping it all to themselves. The results are published in Nature1.
Although this might seem an obvious finding, it's important to confirm anecdotal evidence with experiments, says Matthias Sutter of the University of Innsbruck in Austria, who studies fairness and trust behaviour in children. "Having children myself, I can confirm [that 3-year-olds are selfish]," he says. "But we don't know when the change happens, or the magnitude of the change."
Several other factors influenced how egalitarian the children were. The team found that children without siblings were 28% more likely to share than children with siblings. On the other hand, the youngest children in a family were 17% less willing to share than children who had only younger siblings. Sutter, who is one of five children, thinks these results make sense: "You have to take care that you get some of the pie," he says. "If you're an only child, there's no need for that."
In addition, if children knew that their partner was from the same playgroup or school (an 'ingroup' member), they were more concerned about being fair, and with age this bias increased. This suggests that being nice to people you know – parochialism – is something that develops alongside a sense of equality, says Fehr.
Chimps versus children
The findings are also interesting from an evolutionary perspective, the team suggests. Similar experiments performed on chimpanzees show that the animals aren't willing to provide food for a partner even if it doesn't affect the food they receive2. "Chimps seem not to care about the other's welfare in this game, whereas children at age eight already care a lot, but children at age three care only a little," Fehr says.
Further work along these lines could reveal more about why humans are so bothered about fairness, and the part this has played in building human societies. Without it, says Sutter, "there would be no education, no socialization, no social norms".
Fehr, E., Bernhard, H. & Rockenbach, B. Nature 454, 1079-1083 (2008).
Silk, J.B. et al. Nature 437, 1357-1359 (2005).
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Smith, K. Children learn rules of equality by age eight. Nature (2008). https://doi.org/10.1038/news.2008.1064