My research goal is explaining the cultural origins of cooperation and conflict, especially in humans. 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.

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.

Interviewing an older Turkana warrior.

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 (I think both).


So far we have interviewed over 150 Turkana warriors about their raiding history, potential PTSD symptoms, and participation in social institutions of Turkana warfare. Based on the gene-culture conflict theory of combat stress, I predict that some of the classic PTSD symptoms will occur with common frequency across human societies and that other classic PTSD symptoms will be more culturally variable.

Interviewing a younger man about his experience with raids.

Interviewing a younger warrior.

Mathematical theories of cooperation and conflict

I have published theoretical papers explaining 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), and how sex-biased patterns of tool use in non-human species can be explained by cooperative teaching (Zefferman 2016, attached). In forthcoming publications I model how cultural evolution influences cooperation and conflict between modern nations (Zefferman & Joyce, in review) and the evolution of hierarchically-organized cooperation (Zefferman, in prep; Zefferman, Robinson, Gavrilets & Turchin, in prep).

I am now applying my theoretical research how we might solve global collective action problems, like climate change. I started tackling this question with an NSF-funded interdisciplinary working group tasked with modeling possible scenarios for transitioning to sustainable societies (link). Drawing on my background in environmental sustainability and dynamic models of cultural evolution, I am collaborating with the team on modelling the cultural and institutional changes may be required for this transition. It is a challenge because the between-group competition that often drives cooperation at smaller scales, is not an obviously viable option for global-scale cooperation. This is an unsolved puzzle and one where I plan to heavily invest in my future research. An abstract of my first paper on this puzzle was recently accepted to a special issue of Sustainability Science and is slated for publication next year.