Image caption Using puppets allows scientists to test how much children would really share with their peers

It looks like a fairground game: a little girl and a puppet play together, using miniature fishing rods to hook tiny buckets of coins.

But this is actually a psychological experiment. Its aim is to measure a very complex human concept - fairness.

The game works like this: the puppet (with the aid of an adult puppeteer) and a three-year-old participant gather their hauls of little buckets. Then the child/puppet team is rewarded with stickers - one for each coin they have collected.

We were very surprised to find sophisticated sharing behaviour already present in three-year-olds Patricia Kanngiesser, University of Bristol Today Programme: What is fairness?Lab UK: Test your own morality

At this point the child has to decide how to share his or her prized stickers with their puppet partner.

This simple game revealed that, by the age of just three, children choose to reward their peers based on merit. The children gave the puppet more stickers if it had "worked harder" - gathering more coins.

Patricia Kanngiesser from the University of Bristol led the study, which was published in the journal PLoS One. She said she was amazed by the result.

"We were very surprised to find this sophisticated sharing behaviour already present in three-year-olds," said Ms Kanngiesser.

"Previous research has found that children don't really begin to share according to merit until they are six years of age and older."

Using puppets allowed the experimenters to carry out a controlled experiment whilst still revealing exactly how the children would behave towards peers in a real world situation.

Innate justice

So, if young toddlers with almost no social experience give up more of their stickers to a deserving peer, does this mean that humans are hard-wired to treat others fairly?

Co-operation and fairness are fundamental aspects of human behaviour Susanne Shultz, University of Manchester

Prof Felix Warneken from Harvard University, a co-author of this study, has been studying cooperative behaviour in children and in chimpanzees for almost a decade.

He said knowing where our concept of fairness comes from - whether it is learned or innate - is "the toughest question".

"We can only rule things out with our experiments," he explained. "So we can rule out that it needs formal education, or that it requires sophisticated reasoning about incentives.

"[Fairness] is something that emerges with children's earliest forms of working and interacting with peers."

Ms Kanngiesser said there was a "natural human predisposition" towards treating others fairly.

"It seems to be intuitive," she said. "People have found that even by 18 months of age, children have expectations about how things should be shared fairly."

And there are logical, human reasons for this natural bias towards fair play.

Dr Susanne Shultz, a researcher from the University of Manchester who specialises in social behaviour in primates, pointed out that fairness was central in long-term stable human relationships.

This and other similar studies, Dr Shultz said, demonstrated that "co-operation and fairness are fundamental aspects of human behaviour".

"[This study] also reframes social intelligence in terms of cooperation rather than deception," she added. "I think that's really nice."

Deception has played a large part in the scientific study of fairness. While this experiment asked children to share the rewards with a partner after completing a shared task, many studies focus on whether humans, and other primates, choose to cheat a partner or to punish others for treating them unfairly.

A classic example of this type of test is known as the ultimatum game.

This is where one participant is tasked with making an offer to share something of value - for example, an amount of money - with a partner.

The partner then has the opportunity to either accept or reject this offer. And this is where the punishment comes in.

If the recipient decides to reject an apparently unfair offer, both participants receive nothing.

"[This new study] is a nice change from looking at social intelligence from a purely Machiavellian perspective," Dr Shultz said.

Uniquely human?

In recent years there has been increased scientific interest in whether non-human primates are able to understand the concept of fair play.

As well as gaining an understanding of the complexities of animal behaviour, these studies are trying to unpick its evolutionary origins.

The first of these was published in Nature in 2003. Sarah Brosnan, currently at Georgia State University, found that capuchin monkeys reacted dramatically to being treated unfairly.

Dr Brosnan trained wild monkeys to work with human handlers on a simple task; the monkey would hand over a piece of rock and, in return, receive a food reward.

In the experiment, two monkeys sat side by side. The handlers either gave both monkeys an identical reward (a slice of cucumber) or gave just one of the monkeys the much preferred reward of a grape for completing exactly the same task.

As it was reported at the time by National Geographic: "Capuchins who witnessed unfair treatment and failed to benefit from it often refused to conduct future exchanges with human researchers, would not eat the cucumbers they received for their labours, and in some cases, hurled food rewards at human researchers."

Since then, several groups have tried to test fairness in our closer and apparently more intelligent cousins - the great apes.

But, Dr Shultz said, overall, "non-human primates, apes included, are not very good at solving fairness problems".

In one study published recently in Biology Letters, chimps and bonobos presented with a food-based ultimatum game would accept any food reward on offer - no matter how meagre their share was in comparison to their partner's.

The apes would also consistently steal food from one another during the task.

So is a sense of fairness; of treating others justly at our own expense, a uniquely human characteristic?

Dr Warneken explained that the scientific evidence pointed to a human-ape divide when it came to "recognising inequity".

"There's evidence that some non-human primates are averse to others receiving more than they receive," he said.

But, he added, there was very little evidence that they were averse to offers that were the other way around - where they received more than their fair share.

"That [latter scenario] is a more solid argument for a true understanding of fairness," he explained. "And it's currently something that has no equivalent in non-human primates."

Dr Kanngiessen concluded: "There is a huge debate about what makes us human: is it language, our ability to care about others."

"But even though animals show so many of these complex behaviours, when it comes to helping others and not receiving anything in return, that seems so far to be something [that's] unique to us."