Religious celibacy brings inclusive fitness benefits. Alberto J. C. Micheletti et al. Proceedings of the Royal Society B: Biological Sciences, June 22 2022. https://doi.org/10.1098/rspb.2022.0965
Abstract: 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.
4. Discussion
Taken together, our analyses show that lifelong celibacy can be adaptive under certain conditions. Men with a monk brother have more children and men who sent one of their sons to the monastery have more grandchildren. These effects are strongly significant despite a three-child policy introduced in this area in the late 1980s. With our inclusive fitness model, we have shown that a substantial minority of men can be favoured by selection to be celibate, when the decision is under parental control and when having monk brothers makes men more competitive, leading to higher reproductive success. These conditions are met in our study population, suggesting that this cultural practice has been shaped heavily by the inclusive fitness interests of the monks' parents.
Monks may be enhancing the reproductive success of their brothers in at least two non-mutually exclusive ways. First, as monks do not inherit wealth from their parents [32,38], having a celibate brother might reduce male competition over family resources. We have shown elsewhere [37] that, in this population, men with a monk brother are wealthier than men with a non-celibate brother. Here, we have found that they also have more children, which reveals a key role for brother–brother competition over family wealth. The possibility that monks can also provide material benefits to their brother's family—and thus increase the couple's overall reproductive success—by exploiting their prestigious positions cannot be excluded, as monks command great respect in these communities [38]. We did not find that men with a monk brother have more children than only sons; our analysis of women's age at first birth suggests that their wives might be having children earlier. Further investigation is required to clarify the avenues through which monks benefit their natal families. In other societies, individuals who engage more frequently in religious acts have been shown to have more numerous supportive relations [48,49]. In our case, the fact that only sons and men with a monk brother have the same reproductive success suggests that no such social network effects are present, or that they are unlikely to be important.
Previous research on celibacy suggested that lifelong abstinence could lead to greater lineage survival [5,22–24]. Our sociodemographic analysis has clearly demonstrated that having a celibate child or sibling can be associated with higher reproductive success. Our results help clarify what conditions are necessary for celibacy to appear and be maintained through kin-selected benefits. Genealogical analyses of Medieval and Early Modern European nobility have shown that more children were directed to religious careers in higher social strata [22] and a comparison of two French noble families has suggested that lineages with more celibates were more likely to persist [23]. Both our model and data have shown that celibacy can appear and be maintained in a society without social stratification and hypergamy, two factors that have previously been suggested to be crucial [22,23,50,51]. It has been argued that psychological reinforcement mechanisms and costly ostracism in the case of abandonment of the monastery are key for religious celibacy to appear and be maintained, and they might be used as proximate mechanisms for parents and religious institutions to enforce their own interests [4]. Census data of Catholic priests in ninteenth century Ireland have shown that families who sent at least one son to the seminary were larger, richer and more likely to own land [24]. By contrast, in the present-day United States, Catholic priests tend to come from larger but poorer families [5]. We did not find an effect of wealth in our population: the number of yaks owned by a household does not seem to mediate the effect of having a monk brother or son on reproductive success. Notice, however, that we used current wealth as a proxy for wealth at the time of the celibacy decision, because our data are not longitudinal.
Our model has shown that selection favours celibacy only if it relaxes competition within the monk's family, not within the wider social group. Just like infanticide by parents did not evolve for population regulation [12,52], committing one's son to religious celibacy cannot be favoured by selection for the ‘good of the group’. Moreover, we have shown formally that parents and offspring are indeed in conflict over religious celibacy, with parents favouring higher levels of altruism, analogously to what has been shown for other behaviours subject to parent–offspring conflict [46,47,53–55]. By developing a demographically explicit model employing the latest inclusive fitness methodologies [25,27–31], we have also clarified that dispersal rates influence celibacy decisions when under individual control (and, since only the celibate's brothers benefit from his altruism, celibacy can be favoured even when males never disperse, cf. [25,45,56,57]; see electronic supplementary material for details). On the other hand, dispersal rates have no effect in the more likely scenario when parents decide. In this case, only costs and benefits matter because parents are equally related to all sons, analogously to what has been shown in models of mother–offspring conflict over offspring size [55].
By elucidating the inclusive fitness costs and benefits associated with celibacy, our analysis has highlighted that parent–offspring conflict over the decision is substantial and that only when parents win that conflict would a reasonable proportion of the population become monks. In our population—and in the context of several other religions worldwide [4]—parents induced sons to become monks at a young age, configuring this behaviour as a form of parental manipulation [12]. Several studies have revealed discriminative parental solicitude in a range of contexts that may share similar patterns to the case studied here. Parents often penalize later born sons in regard to care and other investments, including wealth inheritance [14–18,58–61]. For example, Gibson & Gurmu [61] have shown that, in Ethiopia, competition between brothers has adverse effects on later born sons when land is inherited from fathers, but not when it is assigned by the government. In our population, as new and more remunerative job opportunities become available in nearby towns and cities, competition between brothers over family resources may be declining, the incentives for celibacy thus decreasing and parent–offspring conflict gradually disappearing—contributing to a gradual abandonment of the practice.
We have shown that religious celibacy can be adaptive: so why is this practice not more widespread? Two non-mutually exclusive reasons exist. First, as we have discussed above, the conditions for it to be adaptive are not met everywhere or—as could be the case for Europe—were once met but are not any longer. The Tibetan plateau is a harsh environment where competition between siblings for parental resources is likely to be high. Second, religious celibacy as a culturally recognized option needs to be available to a population for it to be adopted as a parental discrimination strategy. In our population, Tibetan Buddhism affords this opportunity. In Medieval and Early Modern Europe, Catholic Christianity also offered this way for parents to suppress their children's reproduction [22,23]. Practices are adopted when they are in line with an individual's interests and, when they are not, they are either abandoned or altered. In this regard, the cases of religions that dropped celibacy requirements for their practitioners—like Protestant Christianity or Japanese Zen Buddhism—are a promising avenue for future research.
Much of the current literature on the evolution of cultural phenomena focuses on transmission biases [62,63], as potential proximate mechanisms for cultural change. However, that framework does not have the power or generality of inclusive fitness theory to help us understand the design and diversity of cultural phenotypes along ecological lines. Humans are strategic in terms of the design or acceptance of cultural traits, adopting those that satisfy preferences that are beneficial to their fitness, as also suggested by recent other work [64–66]. So inclusive fitness is still a framework that potentially has predictive power with respect to the design of cultural phenotypes. Behavioural ecology models have long been used to increase our understanding of the diversity of human behaviour, including cultural behaviour [42], and here we have shown that inclusive fitness interests appear to play a role in shaping both parental preferences and the design of a costly religious institution. Inclusive fitness can help us to make predictions about the phenotypes of cultural institutions that develop in human populations [42].
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