This seems like a pretty good use of evolutionary theory to understand a cultural practice. What are your thoughts?
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. Grandparents with a monk son also had more grandchildren, as their non-celibate sons faced less or no competition with their brothers. 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.
This hints that celibacy can evolve by natural selection. 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.
In the model, we show that celibacy becomes much more common only if it is the parents who decide it should happen. Parents gain fitness from 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 faced dishonour if they later abandoned their role, suggests a cultural practice shaped by parental interests.
My thoughts exactly. I almost mentioned evolutionary psychology myself. The assumption that all traits are both beneficial and selected for is not supported by real world data. Neutral traits can reach fixation, and a mutation can be selected for if it has beneficial effects that outweigh any deleterious effects it may also have. For all we know, celibacy is a deleterious side effect of a mutation that confers much greater benefit elsewhere, and that’s assuming celibacy is a heritable trait to begin with.
Pretty sure celibacy is not heritable. Though it’s true that if your parents never had children, it’s very likely you won’t either. What would have to be heritable is the inclination to make one of your children a monk. And I sincerely doubt there’s an allele for that in the Tibetan population.
The suggested mechanism appears to be that sending a (younger) son into the monastery means there’s more of the family’s resources for the remaining (elder) son to inherit, leading to increased resources to support a larger family.
However I cannot see how the following snippet fits into that explanation:
… sisters-in-law of monks have their first child 1.30 years earlier than women who do not have a monk brother-in-law …
This makes me wonder if there might be a missing variable, that is correlated to both sending sons into the monastery, and to younger marriages (and thus first births). If such a variable exists, then the question would then become whether the omission of this variable has biased the results for sending sons into the monastery?
It would seem at least possible to me that more traditional Tibetan families (i) marry at a younger age, (ii) are more likely to send sons into a monastery, and (iii) more likely to have larger families, than less traditional ones – and that sending sons into the monastery may in fact be more of an endogenous variable than an exogenous one.
Admittedly it has been several decades since I studied these sorts of issues in university, so I cannot remember what the researchers should have been doing to allow for this possibility (and therefore whether they have already done this).
Sure. But cultural evolution isn’t what we mean when we just say “evolution”. The title is still deceptive.
Nor is there any evidence here that tendency to make a child into a monk is inherited, even culturally, within Tibetan society; that is, no evidence that there is inherited cultural variation for such a tendency.
Or it could just have incomplete penetrance. But you will note, once more, that it isn’t the individual who makes the decision to be celibate; it’s his parents. So it has to be the tendency to force celibacy on others that’s the inherited trait.