A study at the Yale Institute of Network Science has found that social networks are critical to humans’ motivations for participation in group violence.

The researchers mapped social connections among 91 men of the Nyangatom tribe, which is known for participating in group raids of neighboring tribes, in Ethiopia over three years. The study found that men were more likely to participate in a raid if they were recruited by a friend rather than recruited by the leader of the raid. These results are relevant to other violent phenomena, such as mobs, revolutionary movements and gang warfare, because they show that people are more likely to participate in risky behaviors if their friends are participating as well.

“What we found is that people go to war with their friends,” said Nicholas Christakis ’84, the co-director of the Yale Institute of Network Science and a senior author of the study.

To determine friendship networks, the researchers mapped a gift-exchange network within the community, Christakis said. The researchers asked each man in the Nyangatom tribe to whom he would like to give a small gift, and these gift networks were then mapped to show the social networks of the individuals within the tribe.

The results found that a Nyangatom man was 19.2 percent more likely to participate in a given raid for every nonleader friend going on the raid, while he was only 6.8 percent more likely to participate if the friend was a leader of the raid.

Additionally, the study found that there was no significant increase or decrease in a man’s likelihood to participate in a raid when a sibling was participating in the raid, nor was likelihood to participate affected by physical characteristics such as height or weight.

“What this suggests is whether you purchase an iPad or whether you read the [Yale Daily News] may have less to do with your gender or your major or which dorm you live in and more to do with what are your friends doing,” Christakis explained. “These phenomena are not fully explainable or even well explainable based on assumptions about your attributes. Instead they may have to do with the structure of the group and what is happening around you.”

These social networks are known to be very similar across human societies, said Alexander Isakov, a co-first author of the study who focused on data analysis. Therefore, the results found in this study could actually mirror what occurs in certain violent groups in western society, such as gang violence or terrorism, he explained.

According to Isakov, the violence caused by these groups and the Nyangatom tribe raids are both similar in that it is not formal military conflicts. He added that these raids also do not have traditional leaders, which forces them to rely on pre-existing ties within the community.

“The social ties that exist in this community, the friendship ties, actually amplify this collective violence,” Isakov said.

Christakis pointed out that this study addressed the “collective action problem,” which deals with how to recruit others to participate in potentially dangerous behaviors, especially when the risks are individual but the benefits are shared by the group.

Christakis added that these social networks obey very particular biological, psychological, sociological and mathematical rules, and the goal of the Yale Institute of Network Science is to understand these rules and how to use them to make changes.

“We are studying how to facilitate global health behavior change,” Christakis said. “How do you create artificial tipping points in developing world villages so if one person starts vaccinating their children the whole village starts vaccinating their children?”

The study also developed a mathematical model for determining the leaders in the raid.