That situation changes, however, if having a brother who is a monk makes men wealthier and therefore more competitive on the marriage market. But while all the men in the village might benefit if one of them becomes a monk, the monk’s decision does not further his own genetic fitness. Monks remaining single means there are fewer men competing for marriage to women in the village. We modelled both the case where the decision to send a boy to a monastery is made by parents, as seems to be the case in our field study, and where a boy makes his own decision. To find out 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 a man’s evolutionary fitness, that of his brothers and of other members of the village. This hints that celibacy can evolve by natural selection. The practice of sending a son to the monastery, far from being costly to a parent, is therefore in line with a parent’s reproductive interests. Grandparents with a monk son also had more grandchildren, as their non-celibate sons faced less or no competition with their brothers. Surprisingly, we also found that men with a monk brother had more children than men with non-celibate brothers and their wives tended to have children at an earlier age. Firstborn sons generally inherit the parental household, whereas monks are usually second or later born sons. As monks cannot own property, by sending one of their sons to the monastery, parents put an end to this fraternal conflict. That’s likely because brothers are in competition over parental resources, land and livestock. But there was little or no benefit for sisters of monks. We found that men with a brother who was a monk were wealthier, owning more yaks.
Wealth is generally passed down the male line in these communities. These villages are inhabited by patriarchal Amdo Tibetans who raise herds of yaks and goats, and farm small plots of land. Painting of a monk in a Buddhist monastery. We reconstructed family genealogies, gathering information about each person’s family history and whether any of their family members were monks. With our collaborators from Lanzhou University in China, we interviewed 530 households in 21 villages in the eastern part of the Tibetan plateau, in Gansu province. But were economic and reproductive considerations also involved? Families typically cited religious motives for having a monk in the family. Historically, up to one in seven boys became monks. Until recently, it was common for some Tibetan families to send one of their young sons to the local monastery to become a lifelong, celibate monk. Now our new study, published in Royal Society Proceedings B and conducted in Western China, tackles this fundamental question by studying lifelong religious celibacy in Tibetan Buddhist monasteries. Others have argued that people ultimately create religious (or other) institutions because it serves their own selfish or family interest, and reject those who do not get involved. Some have suggested that practices that are costly to 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.
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