Religious celibacy brings inclusive fitness benefits

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.

No evidence that religious celibacy confers inclusive fitness benefits: a comment on: ‘Religious celibacy brings inclusive fitness benefits’ Micheletti et al. (2022)

“…Micheletti et al [1] argue that the seemingly maladaptive cultural practice of religious celibacy may be advantageous through inclusive fitness. They support this claim with three findings from an agropastoralist Buddhist population in western China: (i) men with a monk brother have more children than men whose brothers are not monks (brother analysis); (ii) women with a monk brother-in-law have their first child earlier than women whose brothers-in-law are all not monks (sister analysis); and (iii) men with a monk son have more grandchildren than men with only non-monk sons (father analysis).…”

“…They support this claim with three findings from an agropastoralist Buddhist population in western China: (i) men with a monk brother have more children than men whose brothers are not monks (brother analysis); (ii) women with a monk brother-in-law have their first child earlier than women whose brothers-inlaw are all not monks (sister analysis); and (iii) men with a monk son have more grandchildren than men with only non-monk sons (father analysis). We believe that these analyses are redundant tests of the same hypothesis and should not be regarded as independent lines of evidence, 1 although they do require separate rebuttals because of idiosyncratic analytic approaches in each case. Micheletti et al also include an inclusive fitness model outlining the theoretical conditions under which religious celibacy can be adaptive and claim that these conditions are met by their studied population.…”

Studying human culture with small datasets and evolutionary models

“…Data files for the sociodemographic study in the original paper are available from the Dryad Digital Repository: [19] (we reduced the number of covariates to protect the identity of the participants). Similarly abridged datasets for the three additional analyses in this reply can be made available.…”

“…In our paper, we aimed [ 1 ] to advance and assess the hypothesis that committing a son to lifelong religious celibacy is an adaptive way for parents to reduce competition between their sons and increase their own reproductive success. We formalized this hypothesis with an inclusive fitness model identifying the conditions under which this behaviour is favoured by natural selection and tested whether these conditions—that is, the assumptions of our model—are met in a Tibetan population where parents often sent a son to the monastery until recently.…”

Post-marital residence patterns and the timing of reproduction: evidence from a matrilineal society

“…In the matrilineal (mostly duolocal) Mosuo, co-resident aunts and sisters were associated with slower reproduction indicating competition between same-sex relatives in the same household [ 58 ]. Competition between brothers has also been demonstrated in patrilineal Amdo Tibetans, with increased wealth and earlier ages at first birth for women married to men with no brothers or non-reproductive brothers who became celibate monks [ 59 , 60 ]. Ultimately, exploring co-residence with parents and siblings is important to better understand reproductive timing.…”