Members of the Turkana, a pastoralist ethnic group in northwest Kenya, have post-traumatic stress disorder symptoms following herding raids and other traumatic events, like those seen in U.S. service members after combat, suggesting PTSD may be a human universal.

The study, published April 13 in Proceedings of the National Academy of Sciences and authored by researchers from Arizona State University and the Naval Postgraduate School, suggests that, although culture may play a role in augmenting one's experience of PTSD, its hallmark symptoms are prevalent across cultures and may have deep evolutionary roots.

A pair of anthropologists, Matthew R. Zefferman and Sarah Mathew, conducted standardized interviews with over 200 members of the Turkana group and compared them with a dataset of PTSD-treatment-seeking U.S. service members who had served in Iraq and Afghanistan. The researchers found that the Turkana exhibited somewhat similar rates of certain PTSD symptoms — including flashbacks, nightmares and heightened startle responses — as compared with U.S. combat veterans. But the Turkana were less likely to experience depressive symptoms as a component of their PTSD. Around 28% of Turkana exhibited symptoms that were severe enough to qualify for a PTSD diagnosis under standards roughly equivalent to those used in the current Diagnostic and Statistical Manual of Mental Disorders, the DSM-5. 

Although the worldwide population is experiencing heightened rates of PTSD, it is especially common in American troops who have engaged in combat overseas, affecting 11-20% of those who served in Afghanistan or Iraq. Yet PTSD in other cultures, particularly traditional pastoralist cultures, has remained understudied. 

The authors chose the Turkana group because of members' frequent and spontaneous exposure to traumatic events. Some Turkana groups are both perpetrators and victims of deadly herding raids, in which men from neighboring tribes attempt to steal livestock and other goods, sometimes engaging in violent battles. As such, the Turkana members are often raised in the midst of warfare, with around half of male mortality caused by violent conflict.

"This is an area in which people are exposed to very persistent and high-stakes risk from being attacked, [even] when they're just going out herding, doing their daily life activities," Sarah Mathew, an associate professor at Arizona State University's School of Evolution and Social Change and the paper's co-lead author, told The Academic Times. "They can be woken up one night with the sound of firearms."

Matthew R. Zefferman, an assistant professor at the Naval Postgraduate School and the corresponding and co-lead author of the paper, incorporated his experiences as an Air Force officer into his fieldwork with the Turkana. "When I started my Ph.D. work, I did reflect on a lot of my military experiences and thinking about warfare and how it works, especially in larger-scale industrial society," he told The Academic Times.

Zefferman spent a total of six months interviewing Turkana warriors in five- or six-hour sessions. He was surprised by their willingness to share intimate details about their traumatic experiences, since this is often a difficult task for U.S. veterans.

The evolutionary roots of PTSD are still unclear, but some researchers have suggested that symptoms like nightmares and flashbacks could help train people to avoid dangerous situations in the future. Similarly, the hypervigilance and sensitivity to threats observed in some people with PTSD may serve as a tactic to react more quickly to threats in the environment.

A society's moral views, the authors noted, may play a role in the degree of depression experienced by people with PTSD. American soldiers' heightened rates of depression, for example, may stem from the stigma they encounter for having engaged in morally ambiguous behavior that is not universally accepted by their peers at home.

And while the Turkana who participate in battle remain a close-knit part of the community, U.S. soldiers are separated from their families and communities during combat and may return home to a setting in which few people can relate to their traumatic experiences. The sharp divide between civilian and military life, as well as the politicization of combat-oriented mental issues like PTSD, can lead to the further ostracization of veterans who arrive home from a combat zone. "They try and talk about this with their family or with their friends back home in the civilian world — this is especially true of reservists who don't come back with their unit — and they feel like people just don't accept it," Zefferman explained. "And they aren't able to discuss these issues."

Turkana warriors are typically greeted with acceptance by fellow members of their group and may engage in rituals before and after violent encounters in order to sanctify and reflect on those traumatic events. For instance, after a warrior has killed someone from a neighboring group in combat, he may engage in a ritual to prevent haunting by the dead person's ghost. These hauntings relate closely to the kinds of immersive, vivid nightmares that are commonly reported in cases of PTSD in the West. Stories of past raids are shared both in formal ceremonies and in everyday conversations. According to the authors, some of these rituals and talks may serve to tamp down the kinds of depressive symptoms that are more common in cases of PTSD in industrialized nations.

Meanwhile, among U.S. service members, "There are often rituals if someone in your unit dies. Or there are awards that have to do with bravery, in which you might have killed, but the killing is incidental," Zefferman noted. The U.S. military could experiment with "having some sort of potentially somber ritual that says, 'Hey, we recognize that there is a moral weight to this'" in order to address difficult, traumatic experiences, Zefferman said.

The researchers cautioned that it is too early to determine the extent to which PTSD may be related to evolutionary biological factors. In particular, a wider variety of societies will need to be examined in order to capture a more holistic view of the cultural factors that may contribute to the disorder.

In future studies, the researchers are interested in determining how elements of authority and agency could play a role in the development of PTSD. In an American context, soldiers may be forced to engage in activity to which they themselves are morally opposed, leading to a greater sense of wrongdoing after the event itself. In Turkana cattle raids, on the other hand, involvement is usually voluntary and improvised, with no central leader or hierarchical moral authority. 

The researchers also hope to further break PTSD into its constituent parts to better understand why some soldiers experience only a subset of its symptoms.

"Our research suggests one of the gaps in the way that PTSD is being studied in the West — that we need to start thinking about potentially PTSD being a mix of different phenomena that have different explanations and different risk factors," Mathew said. "And, at least according to our hypothesis, it's possible that the depressive symptoms might have more to do with people's perceptions about whether their society thinks what they're doing is wrong."

The study "Combat stress in a small-scale society suggests divergent evolutionary roots for posttraumatic stress disorder symptoms," published April 13 in Proceedings of the National Academy of Sciences, was authored by Matthew R. Zefferman, Naval Postgraduate School and Arizona State University; and Sarah Mathew, Arizona State University.