Monkeys show prejudice too

Yale psychologist have discovered that rhesus macaques judge members of different social groups similarly to humans.
Yale psychologist have discovered that rhesus macaques judge members of different social groups similarly to humans. Photo by Laurie Santos.

While stereotyping and the “us” versus “them” mentality of high school cliques might seem to be a uniquely human phenomenon, a new study by Yale psychologists suggests that human prejudice may have a much longer evolutionary history.

The researchers, led by Neha Mahajan ’06 GRD ’11 and psychology professor Laurie Santos, found that rhesus macaques — monkeys that branched off from the human evolutionary line over 25 million years ago — make snap judgments about different social groups in much the same way that humans do. Rhesus monkeys can quickly differentiate between members of ingroups and outgroups, and they evaluate ingroup members positively while viewing outgroup members negatively, the researchers reported in the March issue of the Journal of Personality and Social Psychology.

“One of the main implications is that group biases are not necessarily as complex and culturally transmitted as we may once have thought,” Mahajan, who designed and oversaw the study, said. “Clearly there are things that happen in our language, society and culture that might exacerbate these biases, but the capacity to have negative attitudes towards one’s outgroup definitely exists without having language or human culture.”

Working with Natashya Gutierrez ’10, Margaret Martinez ’10 and Santos, who is the director of undergraduate studies for the Psychology Department, Mahajan tested monkeys living in Cayo Santiago, an island off the coast of Puerto Rico. The site was ideal for the study because macaques there naturally form different social groups, and a research center on the island maintains detailed records of each monkey’s group membership, Santos said.

Throughout the study, the researchers used preferential looking tests to assess monkeys’ reactions to stimuli. The logic behind such tests, which have been used with primates and human infants, is that a subject will tend to look longer at something that is unexpected or scary, Santos said.

“We had some pilot work showing that if you introduce the monkeys to a puppet that behaves nicely and a puppet that behaves badly, they stare longer at the puppet who behaves badly,” Santos said. “We also know that monkeys will spontaneously look longer at negative things like spiders than they will at positive things like fruits. So increased looking time in the monkeys seems to go with things that are negative, that they’re worried about.”

In a first set of experiments, the researchers showed monkeys two photographs, one of a monkey from the subject’s own group and one of a monkey from an outgroup. The monkeys gazed longer at the outgroup photograph, indicating that they could distinguish between members of different groups and that they viewed outsiders more suspiciously.

This result held true even when the experimenters controlled for familiarity, Mahajan said. When subjects saw a picture of a monkey that had just transferred out of their group and one of a monkey that had recently transferred in, they still looked longer at the “outsider,” even when that outsider had been part of their group for years.

“It’s akin to [seeing] somebody who went to your high school, whom you knew all through grade school and high school, and then you go to Yale and they go to Harvard and you say, ‘Oh! You’re [part of] an outgroup,’” Santos said. “You don’t maintain any of this old shared grouping, which is sort of funny.”

Monkeys also formed spontaneous associations between ingroup and outgroup members and arbitrary foam objects created by the experimenters. To test monkeys’ ability to form these associations, the researchers showed monkeys a photo of an ingroup monkey looking at one object and a photo of an outgroup monkey looking at another object, Mahajan said. When they removed the photos of the ingroup and outgroup monkeys, leaving just the objects, monkeys showed increased vigilance toward the object that had been paired with the outgroup. Mahajan compared this effect to the sort of automatic associations humans make when they see a symbol, such as a swastika, that they associate with a certain group, in this case Nazis.

The researchers also developed a nonverbal version of the Implicit Association Test (IAT), a measure of unconscious attitudes and biases, to see whether monkeys would hold similar biases to humans against outgroups, Mahajan said. They showed monkeys sequences of photos that paired an ingroup or outgroup monkey with either fruit, which monkeys view positively, or with spiders, which they fear. Monkeys looked longer at the inconsistent sequences — the pairings of ingroup members with spiders and of outgroup members with fruits — suggesting that these pairs violated the monkey’s implicit attitudes toward the two groups.

But in a twist that the researchers called surprising, only male macaques showed this effect. Females, while being able to distinguish ingroup from outgroup members, did not significantly seem to show negative attitudes toward the outgroup in the association test. This could be because group membership in rhesus macaques passes down the female line, meaning that females never have to fight for their group status. Alternatively, males across species might evaluate outgroups more negatively, Mahajan and Santos said, adding that they may explore the issue further by testing other primate species.

But the fact that humans are evolutionarily predisposed to form group stereotypes does not make prejudice inevitable, the experimenters said.

Both Mahajan and Santos highlighted the monkey’s ability to update their notions of group membership in a flexible manner.

“The big message I take home is that at least some of these processes are really old, but we share in these processes a certain amount of flexibility, even with the monkeys,” Santos said. “And the idea is maybe we can harness that flexibility to be a more tolerant species.”

Richard Wrangham, a primatologist and professor of biological anthropology at Harvard University, said the results of the study fit well with the observed behavior of primates in the wild, in which they seem to classify others into ingroups and outgroups.

“One of the most interesting conclusions is that the kind of cognitive reactions that humans show to members of neighboring groups have now been found in species that do not have the high-level mental abilities that some people have previously hypothesized to be necessary for negative intergroup biases to emerge in humans,” he wrote in an email.

Susan Fiske, a social psychologist at Princeton University, agreed.

“This research is groundbreaking because it suggests the early evolution of ingroup favoritism and outgroup vigilance,” Fiske said. “Complex ideology is not needed for intergroup tensions.”

Psychologists Mahzarin Banaji of Harvard University and Gil Diesendruck of Bar-Ilan University were also co-authors of the paper.

Comments

  • pikadot

    This has nothing to do with “evolutionary history”. Monkeys are just as evolved as the rest of us. The claim in the tag line needs to be substantiated.

  • sah107

    Pikadot, you’re missing the point. The article isn’t arguing that monkeys themselves are evolutionarily ancient, but that these macaques share an evolutionary history with humans. The fact that not only the same group behaviors, but also the same cognitive processes underlying those behaviors, are found in both species, suggests that their common evolutionary ancestor also shared those processes. And THAT suggests that these capacities are over 25 million years old.