Kids share, chimps stash

A legacy of foraging together may prod humans to divvy up spoils evenly

Young kids have no problem saying mine and gimme. Yet even greedy rug rats go out of their way to share cool stuff equally if they’ve worked together to get it, a new study finds.

SHARE ALIKE Pairs of kids as young as 3 years old frequently divvy up toys, but only after working together to get the goodies. Max Planck Institute for Evolutionary Anthropology
GET YOUR OWN Chimps rarely divide food into fair shares after working together to get it, unlike humans as young as 3 years old. Here, two young chimps chow down at a research facility. Felix Warneken

Adult chimpanzees, on the other hand, show no affinity for meting out fair shares after cooperative projects, say psychologist Katharina Hamann of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, and her colleagues.

A tendency to share and share alike evolved in ancient human foraging groups organized around collaborative food gathering, the researchers propose online July 20 in Nature.

“Humans, unlike great apes, may have undergone a period of selection for individuals who divided spoils equally after a collaborative hunt,” Hamann says.

Male chimps sometimes form groups to hunt monkeys, but then individual chimps often keep meat to trade for sex with females (SN Online: 4/7/09).

Some other animals, such as dogs (SN: 1/3/09, p. 13) and capuchin monkeys (SN: 9/20/03, p. 181), will protest unequal deals that don’t involve collaboration, signaling a basic sense of fairness.

Hamann’s team shows that pairs of 3-year-olds are most likely to share with each other after working together, but the jury is still out on chimps, remarks psychologist Frans de Waal of Emory University in Atlanta. In the new study, pairs of apes — with each animal in a separate room — had to pull two ends of a rope simultaneously to move within reach a grape-bearing platform mounted between the rooms, a task that may have confused them, de Waal says. He and a colleague have found that pairs of capuchins share unequal amounts of food after jointly pulling a food tray close (SN: 4/8/00, p. 231).

Hamann responds that chimps thoroughly grasped how to use the test device.

Finding that young kids who work together share together is “a very important result,” says anthropologist Kim Hill of Arizona State University in Tempe. In modern-day human foraging groups, sharing often involves exchanging goods and services considered to be of equal value, such as one person gathering berries to share with another person who provides firewood, rather than people working on one task together. Hill predicts that even 3-year-olds — but not chimps — are capable of this kind of bartering.

The new investigation examined whether adults’ keen interest in fair shares emerges early in life and occurs in humanity’s closest evolutionary cousin, chimps. Pairs of 2- and 3-year-old German children jointly pulled a rope to move a board or a block that pushed four marbles within reach. One child ended up with three marbles and the other with one marble. On about three-quarters of these trials, lucky 3-year-olds gave a marble to their partners to even the take. One-quarter to one-half of lucky 2-year-olds gave a marble to a helper.

A minority of 3-year-olds shared an extra marble after manipulating a block on their own next to another child doing the same, or after being given more marbles than another child. Chimps rarely redistributed uneven amounts of food, whether or not pairs had worked together.

Similar tests need to be conducted with 3-year-olds in non-Western cultures, especially foraging groups, Hamann says.

Bruce Bower has written about the behavioral sciences for Science News since 1984. He writes about psychology, anthropology, archaeology and mental health issues.

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