The first real interview I ever did was in 1999 in Dubrovnik, Croatia, the white-stone city perched on the Adriatic Sea that is now famous as King’s Landing in the TV series “Game of Thrones.” But less than 10 years before, that beautiful city had been the site of a siege, in a war that turned the former Yugoslavia against itself.

I had stepped into a Serbian Orthodox Church, cool and dark against the bright sun outside. The church’s caretaker, an older man with a massive white mustache, told me how the city had descended into chaos during the conflict, which pitted mostly Catholic Croats against mostly Orthodox Serbs and Muslim Bosniaks in the early 1990s. Former friends and neighbors spurned one another until there was no place in the community where the Orthodox caretaker and his Catholic wife were accepted, he said, with tears falling. The church was still damaged from the bombings.

One of the great mysteries of ethnic conflict is how it can suddenly and drastically cleave into groups people who formerly considered themselves one community. In the former Yugoslavia, in Rwanda, ethnic differences that had faded into the background as different groups lived together, intermarried and took part in the same country suddenly surged to the front as conflict began. Ethnic difference became a matter of life and death.

That psychological distancing of one's own people from another ethnic, religious or cultural group is often something that is done deliberately — embraced by leaders who would lead people into conflict. And a new study suggests that the practice is effective: emphasizing essential differences between one's own people and another group does increase enmity, encourage aggression and stir support for policies that promote war.

The research, led by Sasha Kimel, a psychologist at Harvard University, looks at how the attitudes and behavior of Jews and Arabs change when they are given information about the group's genetic similarities or differences. The first three experiments all focused on Americans, while the fourth was done in Israel. Using experiments inside the laboratory and out, the research shows that emphasizing genetic differences created more hostility and aggression between Jews and Arabs, while emphasizing genetic similarities had the opposite result.

Studies of the actual genetic relationship between Jews and Arabs have turned up wildly different results, says Kimel. Some have argued that there is a substantial overlap in genetics, while others say the groups are genetically distinct. Kimel says there are likely multiple ways to measure genetic difference, and that people sometimes seize opportunistically on studies that coincide with their political message. Today, some biologists and anthropologists see race in general as a classification that has little valid genetic or biological meaning.

Kimel says that studying how ideas about genetics affect us is particularly important, since people today are so fascinated with genetics and see it as a powerful and fundamental part of who we are. “Especially when the media is reporting on [genetic difference], it can be really harmful, because it’s easy to misconstrue the results, or to fail to emphasize that we are all very genetically similar,” she says.

In the first study, the researchers had Jewish and Arab study participants read one of two news articles — one that described Jews and Arabs as genetic siblings, and one that described them as genetic strangers. Subsequent tests showed that those who read the article describing Arabs and Jews as genetic siblings showed less explicit antipathy and negativity toward the other group compared with those who had read about the groups being genetic strangers.

In the second study, the researchers added an element to see if changing attitudes would actually affect physical aggression. They performed the first test again on solely Jewish test subjects, except now they had subjects also compete with a randomly chosen opponent with the surname “Mohammed” in a task that would deliver loud blasts of noise to the loser. The participants could set the level and duration of noise that their opponent would receive.

They found that the Jewish participants who had read the article about their genetic similarity with Arabs chose to punish the imaginary Arab opponent with less intense blasts of noise, compared with those who had read the other article. The researchers say the results indicate that awareness of genetic similarity and difference has a direct link to aggression. (There weren't Arab participants in this study or the ones that follow.)

Now the researchers had evidence that those who thought about genetic similarities thought and behaved in a more peaceful manner. But they still weren't clear whether thinking about similarities, or differences, or both, was causing that effect. So in a third study, they introduced a control article about a totally unrelated subject.

Jewish Americans recruited online once again read one of the news articles, and then gave their opinions on various policy actions in Israel. The researchers found that those who read about genetic differences didn't express opinions significantly different from the control group, who read an article about an unrelated subject. However, those who read the “genetic siblings” piece were more likely to agree with the idea that Israel should pursue diplomatic negotiations with the Palestinians, instead of aggressive actions — suggesting that information about genetic similarities could have an impact on the kind of political views that matter for sustaining a war, says Kimel.

For the last study, the researchers looked beyond the United States to Israel, where negative attitudes about Jewish-Arab relations tend to be much more deeply entrenched. They carried out a field study in which Jewish participants on commuter trains traveling between northern and southern Israel filled out a survey in exchange for chocolates.

The study participants again read one of the three articles — about genetic similarity, genetic differences or something else entirely. The researchers then measured their support for positions like political compromise to achieve peace, politically excluding Palestinian citizens, harming Palestinians to achieve military goals and their hope for the conflict.

The chart below shows those results. Here, those who read the article about genetic siblings expressed emotions that weren't significantly different than the control group. But reading the article about genetic strangers significantly worsened negative or aggressive attitudes, like support for collective punishment or political exclusion, and decreased hope and other constructive emotions.

Overall, the research indicates that people who learn about genetic similarities between conflicting groups tend to support policies favoring a peaceful compromise more than those who do not. Yet the study in Israel showed a more pessimistic picture, says Kimel. There, information about genetic differences appeared to worsen negative and aggressive attitudes, while information about genetic similarities didn't do much.

The results suggest that teaching people about their genetic similarities might only make a difference in more peaceful contexts, while in more violent contexts, highlighting the differences may do more to change people's attitudes and behavior, the researchers say.

The study looks at attitudes toward the relationship between Jews and Arabs in the Middle East, one of the world’s longest lasting and most entrenched conflicts — in part a personal choice for Kimel, whose grandmother was a prisoner at Auschwitz. But Kimel says the findings are relevant for many other conflicts around the world.

She points out that ideas of genetic difference have been used to stir up support for war or genocide in recent conflicts. A year before the genocide began in Rwanda, a local magazine cited “specialists in human genetics” in making the outlandish claim that Tutsis were a small minority because they could only reproduce with one another. And before the bloody ethnic conflict in Bosnia, a Serbian nationalist leader expressed his regret at intermarriages between Serbs and Muslims, which would lead to the genetic “degeneration of Serb nationality.”

Kimel says that organizations that monitor conflict should be on guard when media messages or other rhetoric begins to highlight genetic difference like this — it may be a sign of conflict to come.

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