The COVID-19 pandemic has brought science into the public eye and to the attention of governments more than ever before. Much of this attention is on work in epidemiology, virology, and public health, with most behavioural advice in public health focussing squarely on ‘proximate’ determinants of behaviour. While epidemiological models are powerful tools to predict the spread of disease when human behaviour is stable, most do not incorporate behavioural change. The evolutionary basis of our preferences and the cultural evolutionary dynamics of our beliefs drive behavioural change, so understanding these evolutionary processes can help inform individual and government decision-making in the face of a pandemic.

Reputational considerations favour cooperation and thus we expect less cooperation in larger communities where people are less well known to each other. Some argue that institutions are, therefore, necessary to coordinate large-scale cooperation, including moralizing religions that promote cooperation through the fear of divine punishment. Here, we use community size as a proxy for reputational concerns, and test whether people in small, stable communities are more cooperative than people in large, less stable communities in both religious and non-religious contexts. We conducted a donation game on a large naturalistic sample of 501 people in 17 communities, with varying religions or none, ranging from small villages to large cities in northwestern China. We found that more money was donated by those in small, stable communities, where reputation should be more salient. Religious practice was also associated with higher donations, but fear of divine punishment was not. In a second game on the same sample, decisions were private, giving donors the opportunity to cheat. We found that donors to religious institutions were not less likely to cheat, and community size was not important in this game. Results from the donation game suggest donations to both religious and non-religious institutions are being motivated by reputational considerations, and results from both games suggest fear of divine punishment is not important. This chimes with other studies suggesting social benefits rather than fear of punishment may be the more salient motive for cooperative behaviour in real-world settings.

The influence of inclusive fitness interests on the evolution of human institutions remains unclear. Religious celibacy constitutes an especially puzzling institution, often deemed maladaptive. Here, we present sociodemographic data from an agropastoralist Buddhist population in western China, where parents sometimes sent a son to the monastery. We find that men with a monk brother father more children, and grandparents with a monk son have more grandchildren, suggesting that the practice is adaptive. We develop a model of celibacy to elucidate the inclusive fitness costs and benefits associated with this behaviour. We show that a minority of sons being celibate can be favoured if this increases their brothers' reproductive success, but only if the decision is under parental, rather than individual, control. These conditions apply to monks in our study site. Inclusive fitness considerations appear to play a key role in shaping parental preferences to adopt this cultural practice.

Why parents in some societies induce some of their sons to become religious celibates is an evolutionary puzzle. Some have speculated that this might be associated with brother competition for family resources. However, the behavioral ecology of monks and the possible links with competition between brothers remain unexplored. Here, we use demographic data from Amdo Tibetan agropastoralists in western China to evaluate what factors determine the probability of becoming a monk and explore the possible association between wealth and having a monk brother. We found that boys with at least one older brother are more likely to become celibate monks. Patrilocal heads of household, who inherit parental property, are more likely to be first-born sons, whereas men who marry uxorilocally, that is they move to their wife’s household, are generally second- or later-born sons. Moreover, we find that men with at least one monk brother are wealthier than men who only have non-celibate brothers. Together, these results suggest that sending a son to the monastery is a way for parents to decrease competition between brothers over family resources. Harsh and resource-limited environments, like the one we consider, can lead to the emergence of communal households, including polyandrous families, which used to be common in Tibetan areas. Directing one son to become a religious celibate offers a potentially effective solution to brother competition in our population.

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