A group of researchers working in 15 different areas across the globe may have answered one of the deeper questions of the human condition — why are we fair to strangers we'll never see again?

That fairness makes possible the large, interconnected, market-based societies that have grown up mostly in the past 10,000 years.

Two rival theories have been put forward as to why. One suggests that we're fair to strangers because we mistakenly treat them like kin, the other that social conditioning makes us this way.

In the journal Science, researchers present evidence that comes down solidly on the side of social conditioning. They found that people who live in small groups and who grow or catch most of their own food don't really care that much whether they're fair or unfair to strangers, or whether a stranger is punished for being unfair.

People who trade for a larger percentage of their daily food and therefore live in more integrated, larger social groups are much more likely to be fair to strangers.

"We think it was really a lot of cultural learning, and it took 10,000 years of cultural evolution to get to the point where you have a well-run society with billions of people," said Joe Henrich, an evolutionary anthropologist at the University of British Columbia in Canada, the paper's lead author.

Religion appears to play a part as well. People who follow tribal religions are also more focused on their kin and friends and don't care too much about fairness with strangers. People who followed the two world religions in the areas studied, Christianity or Islam, were more likely to be fair to strangers and to want to punish unfairness.

The researchers used games to measure how fair a given person tends to be in interactions where they don't know or aren't related to the person they're dealing with. They also looked at how willing the person was to punish someone else who behaved unfairly.

They played these games in 15 different places around the world that varied in how much they participated in a market economy and how much they participated in world religions such as Christianity or Islam. Sites included Ghana, Papua, New Guinea, Tanzania, Siberia, Kenya, Columbia, Ecuador, Bolivia, Missouri and Fiji.

The researchers measured how integrated an individual was into a market economy by calculating the percentage of calories their family ate which were purchased rather than grown, hunted or fished and then averaged them for a community measure. For religion, they asked if the person followed either Christianity, Islam, a tribal religion or no religion.

People in fully market-integrated societies who practice a world religion were much more likely to make fair offers to strangers in the games.

The researchers suggest religion may have evolved as societies grew bigger because it facilitated larger-scale interactions. Reputation-based social systems start to break down when groups living together get large. Religions internalize norms for behavior toward strangers, making external force less necessary and smooth interactions more possible.

"Small-scale societies typically don't have high gods that are very powerful. There's no notion of heaven or hell. They're not incentivizing proper behavior, how you should treat non-kin, stealing and murder, the kinds of things you need to have a harmonious and moral society," Henrich said. But if people place faith in high gods and the gods are concerned about moral behavior, about lying and stealing, it adds social and cultural pressure that reinforces those behaviors.

In terms of punishment, the smaller the community the person lived in, the less likely they were to want to punish someone for being unfair in how they played the game. Small hunting-band-sized groups were the least likely to punish.

Without social norms that make fairness a given, humans don't seem to do well in social groups over about 300 people, Henrich said. Or at least in New Guinea, where it's been found that when villages get above 300 people, they typically fracture and break into smaller groups under that number. "The usual reciprocity and kinship fall apart and there's no larger entity to oversee those conflicts, so they fracture," he said.

Children certainly aren't born with a tendency toward fairness. In large, complex societies, they're quite selfish until around the age of 12, when they begin to be more adult-like, Heinrich said. But even then, the truly adult sense of fairness doesn't actually come until age 25. "So, it's a long socialization process."

One thing this means is that the assumption that markets run because people are selfish doesn't quite work. Actually, Henrich said, markets run best because people have some motivation toward fairness and equality.

"If you have fully selfish agents, markets don't work because people can't trust each other," he said.