'Even kids know what's fair play'

The study by a team of international researchers found that children as young as three years prefer to share with a peer the spoils after they worked together to earn a reward, even in situations where it would be easy for one child to keep all of them for himself.

The study published in Psychological Science, was inspired by work in chimpanzees that found their cooperation regularly breaks down.

"Chimpanzees often compete over food, which prevents them from working together on a task, even if that's the only chance for them to get a reward" said study author Felix Warneken of Harvard University.

"So we were wondering if the same is true also in young children," said Warneken who carried out the study with researchers from the University of Gottingen and the Max Planck Institute for Evolutionary Anthropology.

Other research in humans has suggested that young kids might not be very good at sharing, but this has usually depended on asking the children what they had do in hypothetical situations, or giving them one shot at dividing up resources.

For this study, the researchers wanted to take a more thorough look at how young children share, particularly in a situation where they had to work together for a common goal.

They had pairs of children, all three year olds, complete a task in which they had to collaborate to get a prize. The prize -- gummy bears, stickers, or other items -- was piled on a board with wheels inside a transparent box.

If only one child pulled on a rope, the board wouldn't move, but if they pulled together, they could bring it toward them and reach the food or toys through windows in the transparent box.

Sometimes there was only one window to reach through; sometimes there were two. But even when there was only one window, which meant that one child could have monopolised the prizes, the children almost always shared equally. Each pair of children was tested several times.

"We were surprised that this rule was so strict—that equality was so strongly preferred," Warneken said, adding that the children shared virtually without conflict.

"It was rarely the case that one took all the resources and the other kid had to say, 'Hey, that's not fair.'"

Sometimes, if one child didn't take their half of the spoils immediately, the other would even point it out.

This helps explain how mutually beneficial arrangements evolve, Warneken said.

It seems like they should show up everywhere -- if we cooperate, we're both better off.

But in fact, even a species as similar to us as chimpanzees doesn't necessarily display that behaviour.

So, maybe what's missing from chimps -- and what humans have in abundance -- is the ability to get along with others, Warneken added.