Young children often protest, "That's not fair." Even very young children understand what is fair. Scientists have run experiments to find out when this starts, and some of the studies start with babies. Infants as young as 12 months expect resources to be divided equally between two characters in a scene. Babies seem to notice unfair sharing. They look longer at unfair outcomes.

A study of 15-month-old infants found that they could identify unequal distributions of food and drink. It also found that this sense of fairness was connected to their own willingness to share. Even infants as young as 15 to 16 months tend to prefer interacting with individuals they have seen behaving fairly. Toddlers under 3 are often reluctant to share. They do not yet understand what sharing means.

By preschool, children will protest getting less than their peers. They will even pay to prevent the peer from getting more. Children apply a strong sense of fairness to others as well as themselves. In one set of studies, children chose to pay so they could stop a selfish child from getting away with unfair behavior. Young children know they should be fair but often still keep more for themselves.

In a 2013 study, Shaw and Olson found that children use the value of resources, not just their number or size, when trying to make outcomes equal. They also report that by age 3 to 4, children share equally with third parties when they can. By age six, they say, children dislike those who share unequally. Children often get better at sharing around age seven or eight.

Young children understand that someone who worked harder may deserve more reward. Even so, young children often prefer to hand out rewards equally, even when one person worked harder. In a study of preschool-age children and others, the four-to-five-year-olds were the ones most likely to prefer that both rewards and punishments be distributed equally. Older children and adults preferred equitable allocations. When asked how to reward a hard worker and a slacker, the most common response from preschoolers was to give one cookie to each.

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. Researchers asked hundreds of children to share stickers. They found that older children seemed to think more explicitly about fairness norms when sharing cost them something. Younger children seemed more preoccupied with their own desires. Young children are aware of fairness norms and see them as applying to themselves, but the weight these norms carry may increase as children get older.

In a study of seven societies, children in every one would give up a food reward to stop a peer from getting more. But children in only three places (the U.S., Canada and Uganda) showed 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. Studies in many places found that children dislike getting less than another child.

"I cut, you choose" 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.

Fairness matters in courts too. In the late 1400s, artists began to portray Lady Justice with a blindfold, but the blindfold was meant to be sarcastic. Lady Justice is often shown with a sword for the power of the law and with scales for two sides of an argument and eventual balance and justice. The Sixth Amendment trial right concerns an impartial jury, also known as a jury of your peers. A guide to teaching young children about voting says the essence of a fair vote is that one person means one vote.

Games need fairness too. Before a football game, the captain announces heads or tails before the referee flips the coin. The toss decides who gets first choice of receiving the kickoff or which goal to defend. Fair play is usually thought of as following the rules. It is deeply rooted in the Olympic ideal and sits at the heart of sports ethics. Greater Good educators note that students who treat others fairly are often more well-liked.

Children learn to share and take turns by playing with other children. A child can keep a few special things just for themselves, and does not have to share everything. A parenting educator argues that real fairness is not about everything being identical but about meeting individual needs.

Words to know

• infants: babies
• preschool: before school
• societies: groups of people
• impartial: not taking sides
• blindfold: eye cover