LETTERS https://doi.org/10.1038/s41562-018-0512-3 1 Department of Human Evolutionary Biology, Harvard University, Cambridge, MA, USA. 2 Canadian Institute for Advanced Research, Toronto, Ontario, Canada. 3 Institute of Economic Studies, Faculty of Social Sciences, Charles University, Prague, Czech Republic. 4 CERGE-EI, a joint workplace of Charles University and the Economics Institute of the Academy of Sciences of the Czech Republic, Prague, Czech Republic. 5 Department of Economics, University of San Francisco, San Francisco, CA, USA. 6 Department of Human Behavior, Ecology, and Culture, Max Planck Institute for Evolutionary Anthropology, Leipzig, Germany. *e-mail: henrich@fas.harvard.edu Does the experience of war increase people’s religiosity? Much evidence supports the idea that particular religious beliefs and ritual forms can galvanize social solidarity and motivate in-group cooperation, thus facilitating a wide range of cooperative behaviours including—but not limited to— peaceful resistance and collective aggression. However, little work has focused on whether violent conflict, in turn, might fuel greater religious participation. Here, we analyse survey data from 1,709 individuals in three post-conflict societies— Uganda, Sierra Leone and Tajikistan. The nature of these con- flicts allows us to infer, and statistically verify, that individuals were quasirandomly afflicted with different intensities of war experience—thus potentially providing a natural experiment. We then show that those with greater exposure to these wars were more likely to participate in Christian or Muslim religious groups and rituals, even several years after the conflict. The results are robust to a wide range of control variables and sta- tistical checks and hold even when we compare only individu- als from the same communities, ethnic groups and religions. What is the relationship between religion and war? Most of the research on this question has focused on the pathway going from religious beliefs and rituals to the kind of solidarity and coopera- tion required for organized, collective action, including conflict 1–9 . Religion’s direct role in organized conflict is not well understood; some evidence suggests that religious commitment can contribute to prejudice and aggression 10–14 while other evidence suggests that various aspects of religiosity can contribute to cooperation and actu- ally attenuate prejudice or even make individuals more amenable to intergroup cooperation 15–17 . Much less attention has been paid 18 to the pathway going in the reverse direction: can the experience of war foster greater ritual participation and religious engage- ment? Here, we focus on this pathway using survey data from three war-torn regions—Sierra Leone, Uganda and Tajikistan—and test whether people who have experienced more war-related violence participate more in religious groups and ritual events. Why would war increase religiosity? Here, we consider two interre- lated sets of hypotheses derived from cultural evolutionary theory 19–21 . First, both theory and evidence suggest that external threats cause people to adhere more tightly to social norms, including their reli- gious beliefs and practices. Recent cultural evolutionary modelling, for example, reveals that potent external threats—including inter- group conflict but also earthquakes, droughts and so on—favour the cultural evolution of both strict norm adherence and harsher pun- ishments for violators because of the central role norms play in in- group cooperation, public goods and coordination 22 . Placed within a broader culture–gene coevolutionary framework 23 , such cultural evolutionary processes would be likely to favour genes that increased people’s facultative or developmental responsiveness to external threats, increasing their sociality, norm adherence and willingness to punish norm violators (potentially signalling norm compliance) 23 . These ideas are broadly supported by descriptive evidence gathered from war combatants 24 , correlational studies 25,26 , laboratory-experi- mental evidence 27–34 and natural experiments 31,35–37 . Some work even suggests that the long-term psychological effects of group threats may be strongest during middle childhood, when many prosocial norms are internalized 36,38 . Under this war–sociality hypothesis, we expect that the experience of violent conflict will increase people’s engagement with religious groups and rituals. Here, any connection between war and religion would merely reflect a more generalized increase in sociality and norm compliance rather than representing something special about religion per se. But, is there anything special about religion besides creating social groups and associations? This brings us to the second set of hypoth- eses. Religions may have culturally evolved to specifically exploit the psychological states created by uncertainty and existential threats as a means to more effectively disseminate themselves. Existing evi- dence suggests that both rituals and beliefs may help people cope with such difficult psychological states. For rituals, much evidence suggests that people may be attracted to rituals or ritualized prac- tices as a means of relieving anxiety or stress and that performing religious rituals may help to mitigate the impacts of traumatic expe- riences on well-being 5,39–43 . Similarly, the prosociality induced by commitments to gods, divine protection and beliefs about life after death may help individuals operate in the face of mortal threats, suffering and existential uncertainty 3,44–46 . Such beliefs may also cre- ate particularly supportive communities or associations that draw in those seeking social connections, charity or mutual aid (as per the above hypothesis) 47,48 . Under this war–religion hypothesis, we expect war experiences to have particularly potent effects on mea- sures of religious engagement and ritual participation. We explore these hypotheses by first establishing if the war– sociality hypothesis applies to religious groups and ritual participa- tion. This extends previous work which has already established that war increases sociality and group participation 35–37,49 . Then, using our detailed surveys, we aim to test the war–religion hypothesis by isolating the unique effect of war on religion over and above that found for non-religious associations, clubs and organizations. Though we cannot explore which specific elements of rituals, beliefs and communities make religions special, our evidence supports both hypotheses. The diversity in locations and nature of the conflicts allows us to assess the robustness of our findings. Moreover, because the data War increases religiosity Joseph Henrich  1,2 *, Michal Bauer 3,4 , Alessandra Cassar  5 , Julie Chytilová 3,4 and Benjamin Grant Purzycki  6 NATURE HUMAN BEHAVIOUR | www.nature.com/nathumbehav