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Chimps found to play fairness game like people

Jan. 14, 2013
Special to World Science  

In some im­por­tant ways, chimps may have more hu­man-like con­cepts of fair­ness than pre­vi­ously rec­og­nized, bi­ol­o­gists say.

In a new stu­dy, sci­en­tists had chil­dren and trained chimps play a ver­sion of two shar­ing games orig­i­nally de­vel­oped to study at­ti­tudes to­ward fair­ness in hu­mans. Both chil­dren and chimps made choices si­m­i­lar in some ways to those of adult hu­man play­ers in the past, the re­search­ers found.

Two sim­ple shar­ing games were the ba­sis of the stu­dy, by a re­search group in­clud­ing prom­i­nent ape ex­pert Frans de Waal of the Yer­kes Na­t­ional Pri­mate Re­search Cen­ter in At­lan­ta. In both games, a first play­er is pre­sented with some mon­ey or oth­er de­sir­a­ble goods. This play­er is in­structed or trained to di­vide the stash with anoth­er play­er, by of­fer­ing that play­er an­y­thing from a small amount to al­most all of it.

In the first form of the game, the “ul­ti­ma­tum game,” the sec­ond play­er must ap­prove of the pro­posed di­vi­sion be­fore it hap­pens; if he or she re­jects it, no one gets an­y­thing.

In the sec­ond form of the game, the “dic­ta­tor game,” no ap­prov­al is nec­es­sary, and both play­ers are simply giv­en their share as pro­posed by the first play­er.

De Waal and col­leagues de­vised extra-sim­ple ver­sions of the games, in part so that the chimps could more easily un­der­stand what was go­ing on. “Both apes and chil­dren re­sponded like hu­mans typ­ic­ally do,” de Waal and col­leagues wrote, re­port­ing their find­ings in this week's early on­line is­sue of the jour­nal Pro­ceed­ings of the Na­tio­n­al Aca­de­my of Sci­en­ces. “If their part­ner's coop­era­t­ion was re­quired, they split the re­wards equal­ly”—but if not, they tended to keep most of the re­ward for them­selves.

In past stud­ies with hu­man adults, dur­ing the ul­ti­ma­tum game, a pro­posed di­vi­sion that's per­ceived as stingy of­ten elic­its an of­fended re­jec­tion from the re­ceiv­er. Pre­sumably to head off such re­jec­tion, pro­posers lean to­ward equal di­vi­sions. But in the “dic­ta­tor game,” where the re­ceiv­er has no say, pro­posers tend to keep more for them­selves.

The si­m­i­lar­i­ties in the ways hu­mans and chimps deal with this situa­t­ion sug­gests “a long ev­o­lu­tion­ary his­to­ry to the hu­man sense of fair­ness,” the re­search­ers wrote. Pre­vi­ous stud­ies have al­so found that chimps and cap­u­chin mon­keys “re­sem­ble hu­mans in their de­ci­sions about coop­era­t­ion and their aver­sion to in­eq­ui­ta­ble re­ward di­vi­sions,” they not­ed.

There was one key dif­fer­ence in the new re­sults com­pared to the past find­ings: nei­ther chil­dren nor chimps ac­tu­ally re­fused any of­fers, de Waal and col­leagues said. “This was likely be­cause nei­ther spe­cies was ex­plic­itly trained that re­fus­al was an op­tion,” they wrote. “Nonethe­less, pro­posers changed their of­fers when a part­ner had con­trol over the re­ward dis­tri­bu­tion.”

“There are sound ev­o­lu­tion­ary rea­sons to ex­pect chim­panzees to be sen­si­tive to un­equal out­comes. They rou­tinely coop­erate by de­fend­ing ter­ri­to­ry, form coali­tions, hunt in groups, share food, and en­gage in re­cip­ro­cal ex­changes that sug­gest men­tal score­keep­ing,” the re­search­ers added. One ad­o­les­cent fe­male chimp, they wrote, was re­cently seen set­tling a fight be­tween two young­er chimps over who would have a leafy branch by split­ting it in two and hand­ing half to each young­ster.

“Chim­panzees are sen­si­tive to un­equal out­comes in ex­pe­ri­ments, re­fus­ing to par­ti­ci­pate when a part­ner earns a bet­ter re­ward for equal ef­fort, and oc­ca­sion­ally even re­fus­ing a bet­ter re­ward when a part­ner re­ceives less,” de Waal and col­leagues went on.

Bi­ol­o­gists be­lieve that when two spe­cies have branched apart ev­o­lu­tion­ar­ily, but share char­ac­ter­is­tic­s—as chimps and hu­mans do—those char­ac­ter­is­tics were likely pre­s­ent in both spe­cies' last com­mon an­ces­tor. That an­ces­tor of hu­mans and chimps, who are some of our clos­est rel­a­tives, is thought to have lived about five or six mil­lion years ago.

In the mod­i­fied ver­sion of the shar­ing games used in the new stu­dy, rath­er than choos­ing among a wide range of pos­si­ble di­vi­sions, the first play­er could choose only be­tween two di­vi­sions: equal, and a “selfish” di­vi­sion fa­vor­ing the pro­pos­ing play­er. This choice was pre­sent­ed in the form of two co­lored to­kens that the first play­er would have to choose be­tween, and which later could be ex­changed for the actual re­wards: food for the chimps, and stick­ers for the chil­dren, who were aged 2 to 7 years.


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In some important ways, chimps may have more human-like concepts of fairness than previously recognized, biologists say. In a new study, scientists had children and trained chimps play a version of two sharing games originally developed to study attitudes toward fairness in humans. Both children and chimps made choices similar in some ways to those of adult people in past studies with the games, the researchers found. Two simple sharing games were the basis of the study, by a research group including prominent ape expert Frans de Waal of the Yerkes National Primate Research Center in Atlanta. In both games, a first player is presented with some money or other desirable goods. This player is instructed or trained to divide the stash with another player, offering anything from a tiny amount to almost all of it. In the first form of the game, the “ultimatum game,“ the second player must approve of the proposed division before it happens; if he or she rejects it, no one gets anything. In the second form of the game, the “dictator game,“ no approval is necessary, and both players are simply given their share as proposed by the first player. De Waal and colleagues devised extra-simple versions of the games, in part so that the chimps could more easily understand what was going on. “Both apes and children responded like humans typically do,“ de Waal and colleagues wrote, reporting their findings in this week's early online issue of the journal pnas. “If their partner's cooperation was required, they split the rewards equally“—but if not, they tended to keep most of the booty for themselves. In past studies with human adults, during the ultimatum game, a proposed division that's perceived as stingy often elicits an offended rejection from the receiver. Presumably to head off such rejection, proposers lean toward equal divisions. But in the “dictator game,“ where the receiver has no say, proposers tend to keep more for themselves. The similarities in the ways humans and chimps deal with this situation suggests “a long evolutionary history to the human sense of fairness,“ the researchers wrote. Previous studies have also found that chimps and capuchin monkeys “resemble humans in their decisions about cooperation and their aversion to inequitable reward divisions,“ they noted. There was one key difference in the new results compared to the past findings: neither children nor chimps actually refused any offers, de Waal and colleagues said. “This was likely because neither species was explicitly trained that refusal was an option,“ they wrote. “Nonetheless, proposers changed their offers when a partner had control over the reward distribution.“ “There are sound evolutionary reasons to expect chimpanzees to be sensitive to unequal outcomes. They routinely cooperate by defending territory, form coalitions, hunt in groups, share food, and engage in reciprocal exchanges that suggest mental scorekeeping,“ the researchers added. One adolescent female chimp, they wrote, was recently seen settling a fight between two younger chimps over who would have a leafy branch by splitting it in two and handing half to each youngster. “Chimpanzees are sensitive to unequal outcomes in experiments, refusing to participate when a partner earns a better reward for equal effort, and occasionally even refusing a better reward when a partner receives less,“ de Waal and colleagues went on. Biologists believe that when two species have branched apart evolutionarily, but share characteristics—as chimps and humans do—those characteristics were likely present in both species' last common ancestor. That ancestor of humans and chimps, who are some of our closest relatives, is thought to have lived about five or six million years ago. In the modified version of the sharing games used in the new study, rather than choosing among a wide range of possible divisions, the first player could choose only between two divisions: equal, and a “selfish“ division favoring the proposing player. The first player would get to choose between two tokens, each of which represented one option, then pass the token along to the second player. The actual rewards were food for the chimps, and stickers for the children, who were aged 2 to 7 years.