Despite all our faults and foibles, human beings are apparently pretty good at sharing and cooperating when compared with other primates. In two recent experiments, 3-year-old children shared prizes - marbles, stickers, or treats - achieved for solving a puzzle, while chimps wouldn't.

The scientists conducting these experiments say they believe such cooperative behavior is a crucial factor in explaining why we've colonized much of the planet and reached a population of seven billion while most other primates are endangered.

We're somewhat brainier than other mammals, but individually, most humans couldn't explain how an automatic transmission works, let alone figure out how to rebuild the global satellite communication system. Collectively, however, we learn more and invent more every generation.

"The big question is why humans are so different from other animals," said Durham University anthropologist Rachel Kendal, an author of one of the studies. "Cumulative culture is one of the relatively few things that seems to be unique to humans."

She and her colleagues put together groups of five 3-year-old children and gave them puzzle boxes that contained stickers as a reward. One of her colleagues showed the children how to "play" with the box so the kids could see there was a reward to be had.

The boxes contained three kinds of stickers requiring three tasks of increasing difficulty. The first involved sliding a door, the second, pushing a button, and the third, spinning a dial. Some postdocs in lab tried it, Kendal said, and reported back that it wasn't easy.

Chimps and capuchin monkeys got the same puzzle but with fruits instead of stamps as a reward. "They were very self-serving," Kendal said, and they focused on getting the treats. Some of the monkeys and chimps reached the second level of the puzzle, and one clever chimp got to the third level.

The children were much more collaborative, she said, helping one another get the boxes open. None of the children got all three levels solved without some input from others. While the children desired the stickers, many voluntarily shared them. Her results were published in Friday's issue of the journal Science.

The other experiment, led by psychologist Michael Tomasello and colleagues at the Max Planck Institute in Germany, showed that children were much more prone to sharing when they'd collaborated on a task. He also used 3-year-olds and chimps, but in his setup a box would release some marbles only if two players each pulled on a rope at the same time. Usually one player would get more marbles than the other, in which case the children usually redistributed them evenly, while the chimps kept whatever was doled out.

Children were no more generous or fair-minded than chimps, however, if they were simply given different numbers of marbles. They also failed to share when the two ropes were independent, so it was clear they were not collaborating to get the marbles out.

Tomasello said his results and those released Friday both help explain why human culture is "cumulative." Since the 1990s, he said, scientists have been noting that different troops of chimpanzees learn and copy various ways of doing things, such as using sticks to get termites out of logs.

But such chimp "culture" is static, he said, while human culture builds on past knowledge. The outlier chimp who figured out the three levels of the puzzle in Kendal's experiment never tried to show any of his or her compatriots how to do it.

Penn evolutionary psychologist Rob Kurzban, who wrote a commentary piece for Friday's Science, said that humans were more prone to sharing than chimps, but that finding doesn't automatically lead to the conclusion that human sharing was responsible or necessary for cumulative culture.

As an analogy he pointed out that members of some bird species can recall the locations of hundreds of seeds, and those birds can also fly, but that doesn't mean the flying ability caused the success at seed-recovery.

Others see more practical applications, extending the results to grown-ups. In an op-ed piece for the New York Times, University of Virginia psychologist Jonathan Haidt noted that most people don't see the economy as a collaborative exercise so politicians will get nowhere trying to persuade people to divvy up their marbles more evenly.

Instead, he argued, leaders might focus on the issue that angers both the tea partyers and the occupiers. "It's not that some kids have more marbles than others. . . . It's that some kids get to rig the marble machine before the rest of us have a chance to play with it."

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