Human behaviors are often explained as hard-wired evolutionary leftovers of life on the savannah or during the Stone Age. But a study of one very modern behavior, fairness toward total strangers one will never meet again, suggests it evolved recently, and is rooted in culture rather than biology.

In a series of three behavioral tests given to 2,100 people in societies around the world, an innate sense of fairness dovetailed with participation in markets and major religions. Generally speaking, these use social norms and informal institutions to promote fairness, which allow societies to become larger and more complex.

Biologically speaking, people in the study weren't fundamentally different from their circa-200,000 B.C. ancestors, or from each other. What differed was their cultural DNA.

"You can't get the effects we're seeing from genes," said Joe Henrich, a University of British Columbia evolutionary psychologist and co-author of the study." These are things you learn as a consequence of growing up in a particular place." The study was published March 18 in Science.

Kindness towards strangers is a baffling human trait, given that strangers appear to have been treated with suspicion and violence for most of human history. Some analyses of mortality in the Stone Age — those 2.5 million years of living in small groups that ended just 200,000 years ago — estimate that one in seven people died in combat.

But something changed. Small, family-based groups came together, forming hunter-gatherer tribes. With the advent of agriculture, tribes gave way to city-states. After that, came nations. Anthropologists say all this was only possible because people were willing to treat total strangers in a manner once reserved for kin.

Some researchers say that shift was rooted in a glitch in humanity's primal circuitry, one that caused people to mistakenly treat strangers as relatives. Others think it's a holdover of Stone Age-style thinking – that deep in our brains we see everyone we meet as part of our tiny family, and can't imagine encountering someone who won't ever be seen again.

That's not what Henrich's team thinks. To them, fairness between strangers at the individual level is what allows social organisms to thrive, and to out-compete more selfish societies. From that perspective, fairness-promoting social norms and informal institutions — markets and religion — are an inevitable evolutionary step. Fortunately for us, they make life gentler.

"Once you get cultural evolution going with any strength, you get the enforcement of these norms." Behaviors interlock in a way that rewards fairness and punishes its violation, Henrich said.

To study this dynamic, Henrich's team had 2,100 people from 15 different societies — hunter-gatherers, marine foragers, pastoralists, horticulturalists and wage laborers — play three variations of a game designed to measure their innate sense of fairness.

In the first, a player is given a sum equivalent to a day's earnings, and told to share as much or as little as they want with a second player. Both are anonymous, so from a purely self-interested perspective, there's no reason to share at all.

In the second variation, the second player decides beforehand which offers they would accept and which they'd reject, but rejection means that neither player gets anything. Self-interest dictates that the second player accept any offer, even the lowest.

In the last variation, a third player receives a sum that can either be kept or spent on punishing an unfair offer from the first player to the second. Self-interest dictates that the third player keep their money, and spend nothing on punishment.

The trend in the responses was clear: When people lived in larger communities, and participated more in markets and religion, they were more willing to share, and more willing to punish selfishness.

In smaller communities, lacking the social norms and informal institutions embodied by markets and religion, people have narrow concepts of fairness, "but they're not for dealing with people outside your sphere. There are no default norms for that. There are norms for fairness, but not the kind that let you build a large, well-running culture," said Henrich.

"These findings call into question the standard assumption in economics that preferences are innate and stable," wrote Karla Hoff, a World Bank economist who was not involved in the study, in an accompanying commentary in Science. "We cannot know for certain how fairly our ancestors in foraging bands behaved," but the findings "bring us a closer understanding," she wrote.

Henrichs suggests that culture evolved toward fairness for hundreds of thousands of years before the advent of agriculture, which in turn fostered stable, ever-larger community structures that further accelerated the cultural evolution of fairness. This could have biological effects, favoring the development of linguistic and cognitive abilities, but the fundamental driver was culture.

"We can't rule out the possibility that there was culture-gene interaction, but all the variation we see could be explained by plain cultural evolution," Henrich said.

Images: 1) Game playing in the village of Teci, on Yasawa Island, Fiji./Robert Boyd. 2) Graph showing the average offer in the Dictator Game, arranged by the degree of test subjects' participation in markets./Science.

See Also:

Citation: "Markets, Religion, Community Size, and the Evolution of Fairness and Punishment." By Joseph Henrich, Jean Ensminger, Richard McElreath, Abigail Barr, Clark Barrett, Alexander Bolyanatz, Juan Camilo Cardenas, Michael Gurven, Edwins Gwako, NatalieHenrich, Carolyn Lesorogol,Frank Marlowe, David Tracer, John Ziker. Science, Vol. 327 No. 5972, March 18, 2010.

"Fairness in Modern Society." By Karla Hoff. Science, Vol. 327 No. 5972, March 18, 2010.

Brandon Keim's Twitter stream and reportorial outtakes; Wired Science on Twitter. Brandon is currently working on a book about ecological tipping points.