Jan. 14, 2013 at 3:12 PM ET
Primate researchers in Georgia have laid out what they say is the best laboratory evidence yet that chimpanzees have a human-style sense of fairness. Other researchers, however, say the study is flawed — and they're sticking to their view that fairness may be a uniquely human characteristic.
The debate focuses on a key question about human evolution: How long ago did our ancestors acquire what Abraham Lincoln called the "better angels of our nature"? These angelic traits — such as altruism, empathy and fairness — manifest themselves in behaviors that can run counter to our own self-interest.
The latest research, published in the Proceedings of the National Academy of Sciences, argues that a sense of fair play may have arisen millions of years ago, before our ancestors split off from the evolutionary line leading to other primates. The study's principal author, Darby Proctor of Emory University's Yerkes National Primate Research Center in Atlanta, told NBC News that the research "opens up the door for exploring the evolutionary roots of fairness in non-human animals."
"We've concluded that chimpanzees not only get very close to the human sense of fairness, but the animals may actually have exactly the same preferences as our own species," co-author Frans de Waal, director of Emory's Living Links Center, said in a university news release.
However, the Emory group's findings run counter to what other researchers have found in their own experiments over the past few years. One of the researchers behind those earlier studies, Keith Jensen of the University of Manchester, has said "our sense of fairness is a derived trait and may be unique to the human race."
Today, Jensen said he and his colleagues had serious reservations about the latest experiment. "I was excited to see a replication of our ultimatum game studies, seven years on, but was disappointed by the results," he told NBC News in an email.
How the experiment was run
The "ultimatum game" is a key concept in all these studies: The concept refers to an arrangement in which one player makes a proposal to another player to split up a reward. For example, a parent may offer six stickers to a little girl, on the condition that she divides the treasure with her brother. A researcher may offer six banana slices to a chimp, on the condition that it divides the goodies with another chimp.
If the shares are totally determined by the little girl, or the first chimp, their offer to the partner tends to be as low as possible. That's what's known as the "dictator game." But if the other partner has the power to veto a deal, it gets more complex. Make too low of an offer, and the partner might reject the proposition out of spite — even though the result is that no one gets a reward. That's the "ultimatum game."
The Georgia researchers ran a variety of games using tokens that could be traded for either equal or unequal shares of stickers (for 20 human children, ranging in age from 2 to 7) and bananas (for six adult chimpanzees). The researchers found that the results of the game were similar for the two groups.
If the one offering the goodies was in full control of the split, that individual kept the lion's share of the goodies. The results were different if the two partners had to agree on the split, however. "Humans typically offer generous portions, such as 50 percent of the reward, to their partners, and that's exactly what we recorded in our study with chimpanzees," Proctor said.
How the debate is playing out
Some questions surround the study: The second partner in the ultimatum game always accepted the offer of a split, whether it was equal or unequal. That applied to the kids as well as the chimps. Such behavior might suggest that the recipients would be happy with whatever they got, and didn't care about the fairness of the deal.
In Jensen's eyes, the fact that none of the chimps turned down an unequal split is a "fatal flaw."
"The ultimatum game hinges on the responder," Jensen said. "If the responder didn't understand the option of refusing, I would simply say the study did not work." Similarly, the children involved in the study may have been too young to understand that they could turn down an unfair deal — something that Proctor and her colleagues admit in their study.
They did report, however, that both the chimps and the children occasionally expressed displeasure about an unequal division. For the kids, it was voiced in complaints such as "You got more than me!" For the chimps, it took the form of spitting water at their selfish partner, or hitting a barrier between their cages.
Proctor and her colleagues cite other studies to back up their claim that chimps are sensitive to unfairness — such as anecdotal, non-experimental reports of chimps negotiating over the division of meat, or leafy branches. But such reports aren't yet rigorous enough to resolve the debate.
Proctor acknowledged that more research will be required to get a firmer grasp on the better angels of a chimpanzee's nature. "We can't be sure what all was going on between the chimps," Proctor said. "That's something we'd really like to explore in the future — how much communication is necessary to convey something to another chimp."
More about the better angels of animal nature:
In addition to Proctor and de Waal, the authors of "Chimpanzees Play the Ultimatum Game" include Rebecca Williamson and Sarah Brosnan.
Alan Boyle is NBCNews.com's science editor. Connect with the Cosmic Log community by "liking" the log's Facebook page, following @b0yle on Twitter and adding the Cosmic Log page to your Google+ presence. To keep up with Cosmic Log as well as NBCNews.com's other stories about science and space, sign up for the Tech & Science newsletter, delivered to your email in-box every weekday. You can also check out "The Case for Pluto," my book about the controversial dwarf planet and the search for new worlds.