A common enemy can bring people together regardless of traditionally divisive factors like race. And a new Yale study suggests that peaceful cooperation can create the same effect.

By simply assigning individuals to groups, the study authors were able to consistently decrease the amount by which people categorized others by their race. For study co-author and professor of psychology at UC Santa Barbara Leda Cosmides, the findings suggest a novel strategy for alleviating inter-group conflict. The findings of their study provide evidence for an “alliance detection system” within humans, a cognitive construct that analyzes patterns of cooperation and conflict.

A 2001 study coauthored by Cosmides and John Tooby, a professor of anthropology at UCSB and co-author of the current study, demonstrated that racial categorization could be decreased through intergroup conflict. In the new study, lead by Yale postdoctoral psychology fellow David Pietraszewski, the researchers showed that peaceful cooperation between groups could also decrease the tendency to categorize people by race.

“There are a lot of science fiction movies where the people of Earth are divided by race and ethnicity, and then there’s a space alien invasion that brings the people together into one cooperative unit,” she said. “Many people think that conflict is necessary to erase social boundaries, but we found that not to be true.”

In the study, subjects were shown photos of individuals paired with quotes that these individuals had made about their work. In one condition, the quotes paired with each individual indicated that they either worked for the charity group Partners in Health, or for the group Habitat for Humanity, while in the other condition, the individuals were unaffiliated with a charity group. Subjects were later asked to attribute the statements they had seen earlier to the people who were originally responsible for saying them.

The errors each subject made indicated how they were categorizing people, Tooby said.

“If you heard a remark by Chris Rock and later misattributed this remark to George Carlin, then that would maybe imply that the category you’re using is ‘comedian’,” he said. “On the other hand, if you were to misattribute this remark to O.J. Simpson, then that might indicate that the implicit tool of categorization is race, and that race is operative in your mind when you are thinking about these people.”

By affiliating the individuals with groups like Partners in Health, the researchers were able to decrease racial categorization and increase categorization by charity group, demonstrating that conflict is not necessary to decrease racial categorization. For males, racial categorization was cut in half, and for females it was eliminated completely, Tooby said.

Whether or not the people in the photos were assigned to Partners in Health or Habitat for Humanity groups had no effect on the amount of gender categorization by subjects. While the visual system processes both gender and race, the fact that race but not gender changed in the group condition suggests the alliance detection system is specific for race. Gender, Cosmides said, is not part of the gender alliance system because people always categorize by gender.

To gather further evidence that visual stimuli are not responsible for categorization, the researchers showed subjects the same photo, except that subjects were wearing bright red and bright yellow shirts instead of gray ones. As expected, subjects did not categorize people by their shirt color.

While these studies demonstrated that we are not destined to categorize people by race, Pietraszewski said race remains a dominant form of grouping. The researchers are trying to understand why race is so important and why people use it as a form of categorization at all. There does not seem to be a biological motivation behind race, since the idea of race varies over time and culture, he said.

“Race is a hallucination in the sense that people don’t biologically come into these categories,” he said. “These categories are social constructions and we’re trying to understand why these categories are constructed.”

This study demonstrated that certain social cues in the environment can fundamentally change the degree to which people are categorized by certain features. Now that the researchers have proven that cooperation is sufficient to reduce racial categorization, Pietraszewski said he is interested in conducting other studies to see if the same effect can be produced by other variables, such as similar ages and opinions.

He is currently working on another paper where subjects are aligned by political party membership, which he believes would decrease the amount of categorization done by race, similarly to peaceful cooperation. Cosmides is interested in the way our alliance detection system appears to be capable of responding extremely quickly to which visual cues are necessary to detect alliances, and which ones are not.

“People assume that you have to remove racial discrimination in order to get people to cooperate with one another, but it’s possible that it may actually be the opposite — that by putting people in situations in which they have to cooperate with one another, where race doesn’t predict who is cooperating with whom, this just causes race to fall away as an important dimension for thinking about people,” said Cosmides.

This study was published online in the journal PLOS ONE on Feb. 10.