My research goal is explaining the cultural origins of cooperation and conflict. I approach this topic using theoretical modeling and ethnographic field research. My most recent research focuses on culture, cooperation and conflict in warfare and how that influences combat stress.

Interviewing a Turkana warrior on the Lotikipi Plain in northern Kenya Рabout 50 km from South Sudan.

Ethnography of combat and combat stress

My current project is understanding the evolutionary origins of the symptoms of posttraumatic stress. Towards this end, I conduct fieldwork with Turkana pastoralists in northwest Kenya.


Interviewing an older man about his experience with raids.

An interview with a former Turkana warrior, Lotikipi Plain, Kenya.

I recently compiled theoretical and empirical evidence that large-scale high-risk human warfare can only be explained by a process of gene-culture coevolution. During this process, I developed the hypothesis that the symptoms of combat stress may result from gene-culture conflict. I am examining this hypothesis with Turkana pastoralists in northwest Kenya. My fieldwork, the first scientific study of combat stress outside of industrialized societies, is designed to resolve long-standing disagreement over whether PTSD symptoms are culturally determined or misfiring genetic adaptations.

Interviewing a younger man about his experience with raids.

An interview with a young Turkana warrior on the Lotikipi Plain in northern Kenya.

So far I have interviews with over 200 Turkana warriors about their combat exposure, combat stress, moral beliefs regarding warfare, moral injury and participation in social institutions of Turkana warfare. I hope that this work will help us understand the interplay between the genetic, cultural and environmental causes of PTSD and moral injury and how we might think of better approaches for treatment.

I have also interviewed over 150 Turkana women about their experiences with raids and combat stress. I have recently discovered that women are involved in the defense of their homes from raiders and have expanded my research to include potential moral injury among Turkana women.

Interviewing a Turkana woman in her home about her experiences with raids and combat stress.

Interviewing a Turkana woman outside of her home about her experiences with raids and combat stress.

In collaboration with the Trumble Lab at in ASU’s Center for Evolutionary Medicine, I have collected saliva samples for hormone analysis from 60 Turkana warriors with various levels of PTSD severity. We hope to determine whether Turkana warriors have similar hormonal correlates with trauma as western subjects.

Collecting saliva samples from Turkana warriors for stress hormone analysis.

Mathematical theories of cooperation and conflict

I have published theoretical papers on the general concepts of culture, cooperation and conflict; for example, how how gene-culture coevolution can explain the origins and persistence of cooperation in large-scale warfare (Zefferman & Mathew 2015), the limits of direct reciprocity models for explaining human cooperation (Zefferman 2014), whether existing mechanisms for fostering cooperation can help solve climate change (Zefferman 2018), and how sex-biased patterns of tool use in non-human species can be explained by cooperative teaching (Zefferman 2016). In forthcoming publications I model how cultural evolution influences international conflict (Zefferman & Joyce, in review), cooperation in global climate change mitigation (Zefferman, in revision), and the evolution of hierarchically-organized cooperation (Zefferman, in prep; Zefferman, Robinson, Gavrilets & Turchin, in prep).