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Human prejudice may date back 25 million years or more, scientists say

March 17, 2011
Courtesy of Yale University
and World Science staff

Like peo­ple, some of our mon­key cousins tend to take an “us ver­sus them” view of the world, a study has found. This sug­gests that the ten­den­cy for hu­man groups to clash may stem from a dis­tant ev­o­lu­tion­ary past, sci­en­tists say.

Yale Un­ivers­ity re­search­ers led by psy­chol­o­gist Lau­rie San­tos found in a se­ries of ex­pe­ri­ments that mon­keys treat mon­keys from out­side their groups with the same sus­pi­cion and dis­like as their hu­man cousins tend to treat out­siders. The find­ings are re­ported in the March is­sue of the Jour­nal of Per­son­al­ity and So­cial Psy­chol­o­gy.

Rhesus macaque monkeys tend to take an “us ver­sus them” view of the world, a study has found. (Im­age cour­tesy D. Ma­es­tri­pieri, U. of Chi­ca­go; home­page im­age cour­tesy of Lau­rie San­tos, Yale U.)


“One of the more trou­bling as­pects of hu­man na­ture is that we eval­u­ate peo­ple dif­fer­ently de­pend­ing on wheth­er they’re a mem­ber of our ‘in­group’ or ‘out­group,’” San­tos said. “Pretty much eve­ry con­flict in hu­man his­to­ry has in­volved peo­ple mak­ing dis­tinc­tions on the ba­sis of who is a mem­ber of their own race, re­li­gion, so­cial class, and so on. The ques­tion we were in­ter­est­ed in is: Where do these types of group dis­tinc­tions come from?”

The an­swer, she adds, is that such bi­ases have ap­par­ently been shaped by 25 mil­lion years of ev­o­lu­tion and not just by hu­man cul­ture.

“The bad news is that the ten­den­cy to dis­like out­group mem­bers ap­pears to be ev­o­lu­tion­arily quite old, and there­fore may be less sim­ple to elim­i­nate than we’d like to think,” San­tos said. “The good news, though, is that even mon­keys seem to be flex­i­ble about who counts as a group mem­ber. If we hu­mans can find ways to har­ness this evolved flex­i­bil­ity, it might al­low us to be­come an even more tol­er­ant species.”

San­tos and mem­bers of her lab stud­ied rhe­sus ma­caque mon­keys liv­ing on an is­land off the coast of Puerto Rico. Mon­keys in this popula­t­ion nat­u­rally form dif­fer­ent so­cial groups based on family his­to­ry. 

The in­ves­ti­ga­tors ex­ploited a well-known ten­den­cy of an­i­mals to stare long­er at new or fright­en­ing things than at fa­mil­iar or friendly things. They showed mon­keys pic­tures of oth­er mon­keys who were ei­ther in their so­cial group or mem­bers of a dif­fer­ent group. They found that mon­keys stared long­er at pic­tures of oth­er mon­keys who were out­side their group, sug­gest­ing the crea­tures spon­ta­ne­ously de­tect who is a strang­er and who is a group mem­ber.

“What made this re­sult even more re­mark­able,” said Neha Ma­ha­jan, a Yale grad­u­ate stu­dent in­volved in the re­search, “is that mon­keys in this popula­t­ion move around from group to group, so some of the mon­keys who were ‘out­group’ were pre­vi­ously ‘in­group.’ And yet, the re­sult holds just as strongly for mon­keys who have trans­ferred groups only weeks ear­li­er, sug­gest­ing that these mon­keys are sen­si­tive to who is cur­rently to be thought of as an in­sid­er or an out­sider. In oth­er words, al­though mon­keys di­vide the world in­to ‘us’ ver­sus ‘them,’ they do so in a way that is flex­i­ble and is up­dat­ed in real time.”

San­tos and col­leagues al­so won­dered wheth­er mon­keys eval­u­ated in­group and out­group mem­bers dif­fer­ently: did they as­so­ci­ate these in­di­vid­u­als au­to­mat­ic­ally with “good” and “bad” re­spec­tive­ly? 

To study this, they de­vel­oped a mon­key ver­sion of a test of de­signed to meas­ure con­cealed bi­as in hu­mans. They showed mon­keys a se­quence of pho­tos in which pho­tos of in­group or out­group mon­key faces were paired with pho­tos of ei­ther good things, such as fruits, or bad things, such as spi­ders. The re­search­ers then recorded the time mon­keys spent look­ing at both kinds of arrangements. 

The mon­keys spent lit­tle time look­ing at se­quences that in­clud­ed in­group faces paired with good things like fruits or out­group faces paired with bad stuff like spi­ders, sug­gest­ing that the mon­keys treated these two kinds of stim­u­li as be­ing sim­i­lar, the in­ves­ti­ga­tors said. But the an­i­mals stared long­er at se­quences in which out­group in­di­vid­u­als were paired with pos­i­tive ob­jects like fruit, sug­gest­ing, the scientists said, that this as­socia­t­ion was un­nat­u­ral to the mon­keys. Like hu­mans, mon­keys tend to spon­ta­ne­ously view in­group mem­bers pos­i­tively and out­group mem­bers neg­a­tive­ly, the sci­en­tists con­clud­ed.

The Yale team’s re­sults sug­gest that the dis­tinc­tions hu­mans make be­tween “us” and “them”— and there­fore the roots of hu­man prej­u­dice—may date back at least 25 mil­lion years, when hu­mans and rhe­sus ma­caques shared a com­mon an­ces­tor.

“So­cial psy­chol­o­gists in­tro­duced the world to the idea that the im­me­di­ate situa­t­ion is hugely pow­er­ful in de­ter­min­ing be­hav­ior, even in­ter­group feel­ings,” said Mahza­rin Ba­naji of Har­vard Un­ivers­ity, a co-author of the pa­per. “Evo­lu­tion­ary the­o­rists have made us aware of our an­ces­tral past. In this work, we weave the two to­geth­er to show the im­por­tance of both of these in­flu­ences at work.”


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Like people, our monkey cousins tend to take an “us versus them” view of the world, a study has found. This suggests that the tendency for human groups to clash may stem from a distant evolutionary past, scientists say. Yale University researchers led by psychologist Laurie Santos found in a series of experiments that monkeys treat monkeys from outside their groups with the same suspicion and dislike as their human cousins tend to treat outsiders. The findings are reported in the March issue of the Journal of Personality and Social Psychology. “One of the more troubling aspects of human nature is that we evaluate people differently depending on whether they’re a member of our ‘ingroup’ or ‘outgroup,’” Santos said. “Pretty much every conflict in human history has involved people making distinctions on the basis of who is a member of their own race, religion, social class, and so on. The question we were interested in is: Where do these types of group distinctions come from?” The answer, she adds, is that such biases have apparently been shaped by 25 million years of evolution and not just by human culture. “The bad news is that the tendency to dislike outgroup members appears to be evolutionarily quite old, and therefore may be less simple to eliminate than we’d like to think,” Santos said. “The good news, though, is that even monkeys seem to be flexible about who counts as a group member. If we humans can find ways to harness this evolved flexibility, it might allow us to become an even more tolerant species.” Santos and members of her lab studied rhesus macaque monkeys living on an island off the coast of Puerto Rico. Monkeys in this population naturally form different social groups based on family history. The investigators exploited a well-known tendency of animals to stare longer at new or frightening things than at familiar or friendly things. They showed monkeys pictures of other monkeys who were either in their social group or members of a different group. They found that monkeys stared longer at pictures of other monkeys who were outside their group, suggesting the creatures spontaneously detect who is a stranger and who is a group member. “What made this result even more remarkable” noted Neha Mahajan, a Yale graduate student who headed the project, “is that monkeys in this population move around from group to group, so some of the monkeys who were ‘outgroup’ were previously ‘ingroup.’ And yet, the result holds just as strongly for monkeys who have transferred groups only weeks earlier, suggesting that these monkeys are sensitive to who is currently to be thought of as an insider or an outsider. In other words, although monkeys divide the world into ‘us’ versus ‘them,’ they do so in a way that is flexible and is updated in real time.” Santos and colleagues also wondered whether monkeys evaluated ingroup and outgroup members differently: did they associate these individuals automatically with “good” and “bad” respectively? To study this, they developed a monkey version of a test of designed to measure concealed bias in humans. They showed monkeys a sequence of photos in which photos of ingroup or outgroup monkey faces were paired with photos of either good things, such as fruits, or bad things, such as spiders. The researchers then recorded the time monkeys spent looking at both kinds of sequences. The monkeys spent little time looking at sequences that included ingroup faces paired with good things like fruits or outgroup faces paired with bad stuff like spiders, suggesting that the monkeys treated these two kinds of stimuli as being similar, the investigators said. But the animals stared longer at sequences in which outgroup individuals were paired with positive objects like fruit suggesting that this association was unnatural to the monkeys. Like humans, monkeys tend to spontaneously view ingroup members positively and outgroup members negatively, the scientists concluded. The Yale team’s results suggest that the distinctions humans make between “us” and “them”— and therefore the roots of human prejudice—may date back at least 25 million years, when humans and rhesus macaques shared a common ancestor. “Social psychologists introduced the world to the idea that the immediate situation is hugely powerful in determining behavior, even intergroup feelings,” said Mahzarin Banaji of Harvard University, a co-author of the paper. “Evolutionary theorists have made us aware of our ancestral past. In this work, we weave the two together to show the importance of both of these influences at work.”