Human Cooperation and Warfare

Human Cooperation

IHO Research Scientist Sarah Mathew leads a team of scientists investigating why humans, unlike other animals, cooperate in groups comprising large numbers of genetically unrelated individuals, and how the evolution of this unique form of cooperation is tied to the origins of moral sentiments, cultural norms and warfare. To address these issues, Mathew combines formal modeling of the evolution of cooperation with fieldwork to test theories of how cooperation is sustained.

She has been running a field project in Kenya among the Turkana, a politically uncentralized pastoral society with limited market integration examining how and at what scale people cooperate when centralized political institutions are absent. Her findings have pointed to the foundational role of informal cultural norms, and thereby the human capacity for cultural transmission, in the emergence of human cooperation. As interethnic warfare is prevalent in this population in the form of cattle raiding, Mathew has also done detailed studies examining why Turkana warriors participate in these high-stakes battles where the costs of participation such as injury or death is borne by the individual, but the gains of victory are shared widely.

Through systematic empirical studies in this unique ethnographic context, Mathew's work has shed light on how and at what social scale humans solve the collective action problem in warfare without hierarchical formal military institutions.

Supervising Research Projects

Michael Baumgarten
IHO affiliated graduate student and graduate student at the school of Human Evolution and Social Change
This US veteran work is Baumgarten's dissertation research project.

Media Coverage: https://azpbs.org/horizon/2025/08/ptsd/

Project Summary:

This research examines how cultural transitions between military and civilian environments influence psychological adaptation and mental health outcomes among U.S. veterans. Through a mixed-methods study of 500 post-9/11 veterans across all major service branches and 100 matched civilian controls, it investigates how variations in military acculturation processes and normative frameworks contribute to the development of Post-Traumatic Stress Disorder (PTSD) and moral injury. The study employs a comprehensive assessment protocol incorporating validated clinical measures, moral reasoning evaluations, and cultural identification scales to examine how divergence between military and civilian normative systems influences psychological well-being of veterans. Building on preliminary data from a pilot study of 149 participants, the study tests three interconnected hypotheses examining: (1) the relationship between moral injury and trauma-related depressive symptoms, (2) the impact of normative divergence on psychological adaptation, and (3) the influence of branch-specific variations in military acculturation on mental health outcomes. The research design combines quantitative analysis of symptom patterns and cultural identification measures with qualitative examination of moral reasoning and decision-making processes. Through systematic comparison of veterans from different service branches and matched civilian controls, it investigates how varying intensities of military acculturation and different institutional normative frameworks generate distinct patterns of psychological adaptation during civilian reintegration. Extending beyond traditional clinical models of PTSD, the study examines how cultural transitions influence psychological well-being, with particular attention to the role of moral injury in mediating trauma responses among veterans without direct combat exposure. The research team combines unique qualifications, including Baumgarten's direct military experience in an elite special operations unit, mentorship from scholars in cross-cultural trauma research, and access to specialized veteran populations and support networks.

Intellectual Merit:
The confluence of military acculturation and veteran mental health presents a critical yet understudied domain in anthropological research. This study advances anthropological knowledge through three key theoretical and methodological innovations: 1) Reconceptualizing military service as an acculturative process; 2) Developing quantitative measures for norm divergence between military and civilian contexts; 3) Creating novel methodological tools for studying institutional cultural adaptation. By systematically investigating how differences between military and civilian values influence psychological well-being, the study evaluates the distinct and overlapping impacts of fear-based trauma responses and moral injury. This can contribute to developing more holistic frameworks to further understanding of veterans' mental health challenges that extends beyond traditional clinical models. By analyzing how different military branches create distinct cultural environments that shape their members' worldviews and behaviors, the study speaks to broader theories about how institutions influence human psychology. The study expands acculturation theory by examining how members of a dominant culture adapt to the highly structured environment of military service. By revealing how people maintain, adopt, or reconcile competing sets of cultural values when transitioning between different social worlds, the study can help understand similar transitions in a wide range of institutional settings.

Broader Impacts:
This research advances understanding of PTSD and moral injury etiology while developing evidence-based strategies for optimizing veteran transitions to civilian life. Baumgarten's involvement with the Warrior-Scholar Project, serving 25 college campuses nationwide, provides direct channels for translating research findings into actionable recommendations for veteran service organizations (VSOs). These insights will be disseminated through academic presentations, conference proceedings, and targeted outreach to veteran communities. The project emphasizes STEM education for undergraduate students, particularly veterans, providing hands-on training in qualitative coding, data analysis, and interdisciplinary research methods.

Team

Notable Publications

Zefferman, M, Baumgarten, M. Trumble, B. and Mathew, S. 2025. Little evidence that posttraumatic stress is associated with diurnal hormone dysregulation in Turkana pastoralists. Evolution, Medicine, and Public Health 13(1) 77–91 https://doi.org/10.1093/emph/eoaf004

Why reciprocity is common in humans but rare in other animals. Mathew, S. 2024. Nature 626, 955-956.

Lenfesty, H., Mathew, S., Fikes, T., Ross, C., Boyd, R. 2024. Third-party arbitration and forgiving strategies increase cooperation when perception errors are common. Proceedings of the Royal Society, B. 291: 20240861

Mathew S. and Zefferman M. 2024. Group-structured cultural selection can explain both war and peace. Behavioral and Brain Sciences. 2024; 47: e18. doi:10.1017/S0140525X23002509

Yan, M., Mathew, S. and Boyd, R. 2023 “Doing what others do” does not stabilize continuous norms, PNAS Nexus 2(3) pgad054, https://doi.org/10.1093/pnasnexus/pgad054

Theiss Bendixen, Aaron D Lightner, Coren Apicella, Quentin Atkinson, Alexander Bolyanatz, Emma Cohen, Carla Handley, Joseph Henrich, Eva Kundtová Klocová, Carolyn Lesorogol, Sarah Mathew, Rita A McNamara, Cristina Moya, Ara Norenzayan, Caitlyn Placek, Montserrat Soler, Tom Vardy, Jonathan Weigel, Aiyana K Willard, Dimitris Xygalatas, Martin Lang, Benjamin Grant Purzycki. 2023. Gods are watching and so what? Moralistic supernatural punishment across 15 cultures. Evolutionary Human Sciences, 5, E18. doi:10.1017/ehs.2023.15

Mathew, S. 2022. Turkana warriors’ call to arms: how an egalitarian society mobilizes for cattle raids. Philosophical Transactions of the Royal Society, B. 377 (1851) 20210144.

Taravella Oill, A.M., Handley, C., Howell, E. K., Stone, A. C., Mathew, S., Wilson, M. A. 2022. Genomic analysis reveals geography rather than culture as the predominant factor shaping genetic variation in northern Kenyan human populations. American Journal of Biological Anthropology https://doi.org/10.1002/ajpa.24521

Benjamin Grant Purzycki, Aiyana K. Willard, Eva Kundtová Klocová, Coren Apicella, Quentin Atkinson, Alexander Bolyanatz, Emma Cohen, Carla Handley, Joseph Henrich, Martin Lang, Carolyn Lesorogol, Sarah Mathew, Rita A. McNamara, Cristina Moya, Ara Norenzayan, Caitlyn Placek, Montserrat Soler, Tom Vardy, Jonathan Weigel, Dimitris Xygalatas & Cody T. Ross (2022) The moralization bias of gods’ minds: a cross-cultural test, Religion, Brain & Behavior, 12:1-2, 38-60, DOI: 10.1080/2153599X.2021.2006291

Vardy, T., Moya, C., Placek C.D, Apicella, C., Bolyanatz, A., Cohen, E., Handley, C., Klocová E.K., Lesorogol, C., Mathew, S., McNamara, S. A., Purzycki, B. G., Soler, M., Weigel, J. L., Willard, A. K., Xygalatas, D., Norenzayan, A., Henrich, J., Lang, M., Atkinson Q. D. 2022. The religiosity gender gap in 14 diverse societies. Religion, Brain & Behavior 12 (1-2): 18-37. Baimel, A., Apicella, C., Atkinson, Q., Bolyanatz, A., Cohen, E., Handley, C., Henrich, J., Klocová, E.K., Lang, M., Lesogorol, C.,

Mathew, S., McNamara, R., Moya, C., Norenzayan, A., Placek, C. D., Soler, M., Vardy, T., Weigel, J., Willard, A., Xygalatas, D., Purzycki, B. 2022. Material insecurity predicts greater commitment to moralistic and less commitment to local deities: a cross-cultural investigation. Religion, Brain & Behavior 12(1-2): 4-17.

Zefferman, M. and Mathew, S. 2021. Combat Stress in a Small-scale Society Suggests Divergent Evolutionary Roots for Posttraumatic Stress Disorder (PTSD) Symptoms. Proceedings of the National Academy of Sciences 118:15 e2020430118.

Boyd, R. and Mathew, S. 2021. Arbitration supports reciprocity when there are frequent perception errors. Nature Human Behavior. https://doi.org/10.1038/s41562-020-01008-1

Zefferman, M. and Mathew, S. An evolutionary theory of moral injury with insights from Turkana warriors. Evolution and Human Behavior 41(5): 341-353 (2020).

Handley, C. and Mathew, S. Human large-scale cooperation as a product of competition between cultural groups. Nature Communications 11, 702 (2020).

Lang, M., Purzycki, B., Apicella, C., Atkinson, Q., Bolyanatz, A., Colleran, H., Curtis, R., Handley, C., Klocova, E., Lesorogol, C., Mathew, S., Moya, C., Placek, C., Soler, M., Stagnaro, N., Vardy, T., Weigel, J., Xygalatas, D., Norenzayan, A. and Henrich, J. 2019. Moralizing gods, impartiality and religious parochialism across 15 societies. Proceedings of the Royal Society, Series B 286 (1898): 20190202

Ghrear S. E., Chudek M., Flung K., Mathew, S., and Birch, S. A. J. 2019. Cultural Variations in the Curse of Knowledge: the Curse of Knowledge Bias in Children from a Nomadic Pastoralist Culture. Journal of Cognition and Culture 9 (3-4): 366-384

S Mathew. 2017. How the second-order free rider problem is solved in a small-scale society. American Economic Review: Papers and Proceedings. Vol 107 (5) 578-81.

S Mathew, C Perreault. 2016. Cultural history, not ecological environment, is the main determinant of human behavior. Proceedings of the Royal Society, Series B 283 (1826).

Richerson, P., Baldini, R., Bell, A., Demps, K., Frost, K., Hillis, V., Mathew, S., Newton, E., Narr, N., Newson, L., Ross, C., Smaldino, P., Waring, T., Zefferman, M. 2016. Cultural group selection follows Darwin's classic syllogism for the operation of selection. Behavioral and Brain Sciences 39, e58

Richerson, P., Baldini, R., Bell, A., Demps, K., Frost, K., Hillis, V., Mathew, S., Newton, E., Narr, N., Newson, L., Ross, C., Smaldino, P., Waring, T., Zefferman, M. Cultural Group Selection Plays and Essential Role in Explaining Human Cooperation. A Sketch of the Evidence. 2016. Behavioral and Brain Sciences 39, e30

Mathew, S. and Perreault, C. 2015. Behavioral variation in 172 small-scale societies indicates social learning to be the main mode of human adaptation. Proceedings of the Royal Society, Series B. Published online 17 June 2015. DOI: 10.1098/rspb.2015.0061

Zefferman M., Baldini R. and Mathew S. 2015. Solving the puzzle of human warfare requires an explanation of battle raids and cultural institutions. Proceedings of the National Academy of Sciences 112(20): E2557.

Mathew, S. 2015. Are Cultural and Evolutionary Views of Human Warfare Converging? A Review of “War, Peace and Human Nature: The Convergence of Evolutionary and Cultural Views, edited by Douglas P. Fry (Oxford University Press, 2015)”. Cliodynamics: The Journal of Quantitative History and Cultural Evolution 6 (1). https://escholarship.org/uc/item/4125090f

Boyd, R. and Mathew, S. 2015. Third-party monitoring and sanctions aid the evolution of language. Evolution and Human Behavior. Published online Jun 23 2015. DOI: 10.1016/j.evolhumbehav.2015.06.002

Mathew, S. 2015. The evolution of human cooperation. International Encyclopedia of the Social and Behavioral Sciences 2nd edition 11: 259-266.

Zefferman, M. and Mathew, S. 2015. An Evolutionary Theory of Large-scale Human Warfare: Group-structured Cultural Selection. Evolutionary Anthropology: Issues, News and Reviews 24(2): 50-61.

Mathew, S. and Boyd, R. 2014. The cost of cowardice: third-party punitive sentiments towards free riders in Turkana raids. Evolution and Human Behavior 35: 58-64

Panchanathan, K., Mathew, S. and Perreault, C. 2014. Explaining group-level traits requires distinguishing process from product. Behavioral and Brain Sciences 37(3): 269-270.

Mathew, S., Boyd, R. and van Veelen, M. 2013. Human cooperation among kin and close associates may require enforcement of norms by third parties. In: Cultural Evolution, ed. P. J. Richerson and M. Christiansen. Strüngmann Forum Report 12, series editor J. Lupp. Cambridge, MA: MIT Press.

Jordan, F. M., C. van Schaik, P. François, H. Gintis, D. B. M. Haun, D. J. Hruschka, M. A. Janssen, J. A. Kitts, L. Lehmann, S. Mathew, P. J. Richerson, P. Turchin, and P. Wiessner. 2013. Cultural evolution of the structure of human groups. In: Cultural Evolution, ed. P. J. Richerson and M. Christiansen. Strüngmann Forum Report 12, series editor J. Lupp. Cambridge, MA: MIT Press.

Bowles, S., Boyd, R. Mathew, S. and Richerson , P. J. 2012. The punishment that sustains cooperation is often coordinated and costly. Behavioral and Brain Sciences 35(1): 20-21

Perreault, C. and Mathew, S. 2012. Dating the origin of language using phonemic diversity. PLoS ONE 7(4): e35289.

Mathew, S. and Boyd, R. 2011. Reply to Baumard and Lienard: Mechanistic accounts need to specify why reputation systems yield cooperative outcomes on observed scales. Proceedings of the National Academy of Sciences 108(39): E754.

Mathew, S. and Boyd, R. 2011. Punishment sustains large-scale cooperation in prestate warfare. Proceedings of the National Academy of Sciences. 108 (28): 11375 – 11380

Mathew, S. and Boyd, R. 2009. When does optional participation allow the evolution of cooperation? Proceedings of the Royal Society, Series B. 276: 1167 – 1174

Boyd, R. and Mathew, S. 2007. A Narrow Road to Cooperation, Science, 316: 1858 – 1859.

Major Funding

Mathew is a recipient of the Andrew Carnegie Fellowship, a $200,000 award given to scholars identified as among the country's most creative thinkers to support research on challenges to democracy and international order.