Young children often protest, "That's not fair." Even very young children understand what is fair. Studies in many places found that children dislike getting less than another child. So what is fair? Let's look at what studies of children and monkeys, and what ideas about cake, games and courts, can tell us about fairness. We will begin with the youngest children of all.

Babies seem to notice unfair sharing: they look longer at unfair outcomes. Toddlers under 3 are often reluctant to share and do not yet understand what sharing means. Young children know they should be fair but often still keep more for themselves. Children often get better at sharing around age seven or eight. Learning to share takes time.

In a study of seven societies, children in every one would give up a food reward to stop a peer from getting more. But in only three places (the U.S., Canada and Uganda) did children show a dislike of getting an unfair advantage, at about age 9 or 10. Some parts of fairness look the same around the world, while others change with local customs.

Now think about work and rewards. Young children understand that someone who worked harder may deserve more reward. Young children also often prefer to hand out rewards equally, even when one person worked harder. Children expect people who do equal work to share the reward equally. Children will sometimes give up their own reward to keep things even with another child. Capuchin monkeys in experiments reacted badly to getting a less-favored reward when a partner got a better one.

Here is an old way to split a cake: "I cut, you choose." It is an old, standard way for two people to split a cake. The person who cuts makes two pieces they think are equal. The other person picks the slice they like best, so neither person envies the other. Cake is not the same all the way through, so two pieces of the same size can be worth different amounts to different people.

Games have fairness rules too. Part of being fair is following the rules. In soccer and in the NFL, the referee flips a coin before the game to decide who gets the first choice. Fair play is usually thought of as following the rules. Fair play includes respecting the referee. It is deeply rooted in the Olympic ideal and sits at the heart of sports ethics.

Groups can decide by voting. In a majority vote, the choice with the most votes decides the outcome. After a vote, someone may be unhappy with the result; that is part of voting. Courts have ideas of fairness too. A common idea of justice is applying the same rules to everyone, consistently. A basic idea of fairness: cases that are alike should be treated alike. In the United States, trial by jury is a right guaranteed in the Constitution.

Children learn to share and take turns by playing with other children. Taking turns is one way to share. Playing simple board games or card games helps children practice following rules and taking turns. When two children share a toy, each gets an equal turn, and setting a timer can show when a turn is over. A child can keep a few special things just for themselves, and does not have to share everything.

Words to know

• societies: groups of people
• outcomes: results
• envies: wants theirs
• majority: most votes
• justice: same rules