Why would anyone join an institution that took away the option of family life and required that they be celibate? Reproduction, after all, is at the very heart of the evolution that has shaped us. Yet many religious institutions around the world require exactly that. This practice has led anthropologists to wonder how celibacy might have evolved in the first place.

Some have suggested that costly practices for individuals, such as never having children, can still emerge when people blindly conform to norms that benefit a group – since cooperation is another cornerstone of human evolution. . Others have argued that people eventually create religious (or other) institutions because it serves their own selfish or family interest, and reject those who are not involved.

Now our new study, published in Royal Society Proceedings B and conducted in Western China, addresses this fundamental question by investigating lifelong religious celibacy in Tibetan Buddhist monasteries.

Until recently, it was common for some Tibetan families to send one of their young sons to the local monastery to become a celibate monk for life. Historically, as many as one in seven boys became a monk. Families generally cited religious reasons for having a monk in the family. But were economic and reproductive considerations also involved?

With our collaborators from Lanzhou University in China, we interviewed 530 households in 21 villages on the eastern part of the Tibetan plateau, in the province of Gansu. We pieced together family genealogies, gathering information about each person’s family history and whether any of their family members were monks.

These villages are inhabited by patriarchal Amdo Tibetans who herd yaks and goats and cultivate small plots of land. Wealth is usually passed down the male line in these communities.

We found that men whose brother was a monk were richer and owned more yaks. But there were few or no benefits for sister monks. This is probably because the brothers are in competition for parental resources, land and livestock. Since the monks could not own property, by sending one of their sons to the monastery, the parents put an end to this fraternal conflict. Firstborn sons usually inherit the parental household, while monks are usually second or later born sons.

Surprisingly, we also found that men with monastic brothers had more children than men with unmarried brothers; and their wives tended to have children at an earlier age. Grandparents with a monk son also had more grandchildren, as their unmarried sons faced less or no competition with their brothers. The practice of sending a son to the monastery, far from being costly for a parent, is therefore in keeping with a parent’s reproductive interests.

A mathematical model of celibacy

This suggests that celibacy may evolve by natural selection. To learn more about the details of how this happens, we built a mathematical model of the evolution of celibacy, where we studied the consequences of becoming a monk on the evolutionary fitness of a man, that of his brothers and other members of the village. We modeled both the case where the decision to send a boy to a monastery is made by the parents, as it seems to be the case in our field study, and the case where a boy makes his own decision.

Monks remaining celibate means there are fewer men competing for marriage with women in the village. But while all the men in the village could benefit if one of them became a monk, the monk’s decision does not favor his own genetic fitness. Therefore, celibacy should not evolve.

This situation changes, however, if having a brother monk makes men richer and therefore more competitive in the marriage market. Religious celibacy can now evolve by natural selection because, while the monk has no children, he helps his brothers to have more. But above all, if the choice to become a monk belongs to the boy himself, it is likely to remain rare – from the point of view of an individual, it is not very advantageous.

In the model, we show that celibacy only becomes much more common if it is decided by the parents. Parents acquire the physical form of all their children, so they will send one to the monastery as long as there is a benefit for the others. The fact that boys were sent to the monastery at a young age, with much celebration, and were disgraced if they later abandoned their role, suggests a cultural practice shaped by parental interests.

This model could also clarify the evolution of other types of parental patronage in other cultural contexts – even infanticide. And a similar framework might explain why celibate women (nuns) are rare in patriarchal societies such as Tibet, but might be more common in societies where women are in greater competition with each other – for example, where they have more inheritance rights (as in some parts of Europe).

We are currently developing new research to understand why the frequency of monks and nuns varies across religions and regions of the world.

It is often suggested that the spread of new – even irrational – ideas can lead to the creation of new institutions as people conform to a new norm. But institutions may also be shaped by people’s reproductive and economic decisions.