Protective techniques: Turkana in our sample had a high prevalence of PTSD symptoms, but those with high symptom severity had lower prevalence of depression-like symptoms than American service members

Combat stress in a small-scale society suggests divergent evolutionary roots for posttraumatic stress disorder symptoms. Matthew R. Zefferman and  Sarah Mathew. Proceedings of the National Academy of Sciences, April 13, 2021 118 (15) e2020430118; https://doi.org/10.1073/pnas.2020430118

Significance: Did PTSD and combat stress evolve as a universal human response to danger? Or are they culturally specific? We addressed this question by interviewing 218 warriors from the Turkana, a non-Western small-scale society, who engage in high-risk lethal cattle raids. We found that symptoms that may have evolved to protect against danger, like flashbacks and startle response, were high in the Turkana and best predicted by combat exposure. However, symptoms that are similar to depression were lower in the Turkana compared to American service members and were better predicted by moral violations. These findings suggest different evolutionary roots for different symptoms which may lead to better diagnosis and treatment.

Abstract: Military personnel in industrialized societies often develop posttraumatic stress disorder (PTSD) during combat. It is unclear whether combat-related PTSD is a universal evolutionary response to danger or a culture-specific syndrome of industrialized societies. We interviewed 218 Turkana pastoralist warriors in Kenya, who engage in lethal cattle raids, about their combat experiences and PTSD symptoms. Turkana in our sample had a high prevalence of PTSD symptoms, but Turkana with high symptom severity had lower prevalence of depression-like symptoms than American service members with high symptom severity. Symptoms that facilitate responding to danger were better predicted by combat exposure, whereas depressive symptoms were better predicted by exposure to combat-related moral violations. The findings suggest that some PTSD symptoms stem from an evolved response to danger, while depressive PTSD symptoms may be caused by culturally specific moral norm violations.

Keywords: PTSDcombat stressmoral injuryevolutionary medicinecross-cultural psychology

Discussion

Our findings demonstrate that combat-related PTSD symptoms are not limited to industrialized societies and can occur even in small-scale societies where warriors are venerated and socially embedded in tight-knit communities. In particular, learning-and-reacting symptoms are potentially evolved responses to acute dangers such as those encountered in combat. These symptoms had high prevalence among both American service members and Turkana warriors. Moreover, among the Turkana, combat exposure and combat outcomes were more consistently associated with learning-and-reacting symptom severity than with depressive symptom severity.

Our findings have implications for understanding the roots of moral injury (59, 71, 72), trauma causedy “perpetrating, failing to prevent, or bearing witness to acts that transgress deeply held moral beliefs and expectations” (ref. 59, p. 695). For example, moral injury can occur when soldiers violate morally held beliefs against killing civilians (73). Moral injury might also be the primary cause of combat stress in drone pilots who, even though they are flying combat missions from a control room far from danger, have a high-definition view of the human suffering caused by their missile strikes (74). Our statistical models suggest a relationship between moral injury and depressive PTSD symptoms in particular. Combat exposure and outcome measures are not as important predictors for depressive symptoms as they are for learning-and-reacting symptoms among the Turkana. Instead, predictors assessing exposure to moral violations as perpetrators or victims and experiencing social sanctions are associated with depressive symptoms. Additionally, having moral concerns for a larger segment of people from the opposing side was more strongly associated with depressive symptoms than with learning-and-reacting symptoms. All of this supports the idea that depressive symptoms may be a response to expected social sanctioning due to moral violations, which is consistent with some evolutionary theories of depression (56, 57). However, it is also possible that depressive symptoms, whatever their cause, may make instances of moral injury more salient to study participants. Additional experimental, longitudinal, and cross-cultural research may resolve the direction of causality.

Consistent with the association in the Turkana between expected social sanctions and depressive symptoms, Turkana warriors with high symptom severity were less prone than American service members to experience some of the depressive symptoms of PTSD. This could be because the actual or perceived social risks of participating in war are lower for Turkana warriors than for American service members. Turkana warriors are venerated and there is widespread support from their community for going on raids and defending the Turkana from raids. They do not expect to face moral disapproval for participating in combat (43) (although they do face moral disapproval for cowardice and can be blamed for the death of comrades). In fact, those who have killed in combat are often celebrated in Turkana society with many warriors undergoing akiger, a ritual that scars the warrior’s body to mark him as someone who has killed. Warriors with akiger scars are highly regarded by both men and women. Additionally, raid participation is high among Turkana men, so warriors are almost always in the company of other warriors with similar combat experiences. Many women and children too have experienced raids by other groups. As such, combat experiences are a commonly shared and a frequent topic of discussion in Turkana society. There is little to no stigma associated with sharing the details of combat (43).

By contrast, in the United States and other industrialized nation states, support for war and those who participate in war is often far from universal, and killing, even in combat, is rarely celebrated. American soldiers fight in foreign countries away from the civilian population and, upon returning, they may perceive disapproval of their experiences and actions from friends and family. Additionally, most Americans cannot relate to the experiences of those who have participated in combat. Consequently, warfare presents a moral conflict because what is considered a soldier’s duty in combat can violate prevailing moral norms within the soldier’s society. American soldiers may therefore have a heightened awareness of potential social repercussions especially as they integrate back into civilian life. Veterans’ support groups and group therapy replicate some aspects of Turkana society by allowing veterans to share their experiences with each other, but Turkana warriors receive stronger signals of social support and understanding from all members of their communities.

Since most PTSD research has not focused on symptom-specific causes, moral injury research is relatively new, and combat trauma research has not taken a functional evolutionary perspective, there has been little attempt to associate depressive PTSD symptoms with moral injury in the Western context. A better grasp of symptom-specific patterns of PTSD in Western military personnel, as we have done with the Turkana, would be useful to further evaluate the proposed theory, delineate what moral injury manifests as, and assess how it relates to PTSD.

The effect of killing in combat on PTSD is more ambiguous in the Turkana than in American service members. While killing in combat is an important contributor to PTSD in American service members who served in Iraq and Afghanistan (70, 75), it was not present in the top models of total, learning-and-reacting, or depressive symptom severity in the Turkana. On average, the direction of influence is to reduce learning-and-reacting symptoms but increase depressive symptoms, opening the possibility that it might be a contributor to moral injury even in a population where killing in combat confers prestige. While this was counter to our prediction, it is consistent with some ethnographic observations. The Turkana, as well as neighboring pastoral groups, have culturally specific idioms of distress associated with killing in the war zone, including perceptions of being polluted, beliefs that killing portends future misfortune, and feeling haunted by the enemy’s ghost, which suggest that killing of enemies is a potentially morally hazardous event (76). Among Samburu pastoralists, war zone mercy occurs even in circumstances where killing of the opponent would be normative, indicating that warriors may feel empathy toward their opponents (76) and can thus perceive killing as morally hazardous.

Our results imply that while killing is potentially morally hazardous across cultures, culturally specific institutions mediate its role in causing PTSD, which clarifies why killing is more risky for American service members than for Turkana warriors. First, norms regarding killing of individuals from the opposing side are less restrictive among the Turkana than in nation-state warfare. Unlike in nation-state warfare, the Turkana have a high level of moral autonomy in who they kill in combat, a pattern noted in other pastoral societies (76). Additionally, systems of social support within Turkana society may help alleviate its moral ambiguity. In particular, the Turkana have three postraid rituals that warriors can engage in that are specifically designated for those who have killed enemies in combat (43). In addition to akiger which is optional, akipur is a purification ritual which is viewed as mandatory for anyone who has killed an enemy in combat to protect them from weakening and slowly wasting away. Another ritual, ngitebus, protects a warrior from the ghosts of slain enemy warriors. It is considered optional, but it is almost always performed preventatively in conjunction with akipur. It can also be performed any time after a haunting occurs. For instance, one warrior, due to repeated hauntings, estimated that he underwent ngitebus 11 times over 20 y. These rituals, which require the participation of other community members, could serve as a cue to warriors that the community views their act of killing as morally acceptable. The lack of such rituals pertaining to killing, especially in populations with expansive moral beliefs and restrictive norms of killing in combat, may contribute to the heightened depressive symptoms and moral injury experienced by US military service members.