War experiences have a long-term effect on human psychology, shifting people’s motivations toward greater equality for members of their own group, according to research forthcoming in Psychological Science, a journal of the Association for Psychological Science.

According to study co-author Joseph Henrich of the University of British Columbia, “these effects have the potential to explain both why conflict sometimes leads to cycles of war and sometimes stimulates nation-building in its wake.”

“Our research shows that exposure to war affects human psychology in specific ways,” says psychological scientist and economist Michal Bauer of Charles University in Prague, Czech Republic. “These ‘war effects’ emerge in the short-term and, importantly, they have long-term impact on psychology if war is experienced during middle childhood and adolescence.”

Based on evolutionary theory, Bauer, Henrich, and colleagues hypothesized that experiences with intergroup conflicts should lead individuals to become more focused on their own group’s well-being, since individual survival is often linked to the fate of the group.

The researchers collected data from 543 children in the Republic of Georgia only six months after the conclusion of a brief but devastating war between Russia and Georgia over South Ossetia. They also collected data from 586 adults in Sierra Leone following an 11-year civil war that ended in 2002, which led to the displacement of much of the population and the deaths of over 50,000 civilians.

At each site, the researchers had participants play games in which they had to choose how to allocate tokens to themselves and an anonymous partner. In some cases, the anonymous partner was from the same village or school (in-group); in other cases, the partner was from a distant village or different school (out-group).

Participants’ experiences with war were associated with sharing choices in both Georgia and Sierra Leone. Those participants who had been affected by war were more willing to sacrifice both their own payoffs and those of the group to reduce inequality if their partner was from the same village or school.

There were no such effects, however, for participants who were younger than six or older than 20 when they experienced war.

“These findings suggest that if the war experience takes place during a sensitive window in development between middle childhood and early adulthood, then it leaves an enduring mark,” says Bauer.

As Henrich points out, recruiting participants who had actually experienced war allowed the researchers to study human nature in the context of real-world conflicts. The research may ultimately help to explain why war can lead to either nation-building or to a perpetual cycle of war:

“When people identify with an in-group that coincides with the state or nation, then nation building can be enhanced,” says Henrich. “This could help to explain America’s ‘Greatest Generation,’ which emerged after World War II.”

“But for people who identify with a subnational identity, such as an ethnic group, war can sow the seeds of future conflicts.”