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The "boy pill", assortative mating, and human evolution

There's a debate underway about whether we should oppose sex-selective abortion because it's abortion or because it's sex-selective. Matthew Yglesias argues for the latter. What if scientists developed a "boy pill" that would make a man produce only Y-chromosome sperm? He suggests that:

if we found out that use of the "boy" pill was extremely widespread, this might still legitimately worry us for three kinds of reasons. One is that widespread use of the boy pill would express the inegalitarian idea that men are more valuable than women. A second is that widespread use of the boy pill would reflect the existence of ongoing inequities in society that make it the case that a male child is more valuable than a female child. The third is that there are plausible reasons to believe that even a relatively small gender gap in the population could have problematic macro-scale consequences for society.
I'm not sure about his first and second points. Yes, the lower the status of women, the more likely parents might be to want a son rather than a daughter, even if they personally deplored discrimination against women. But would a male-biased sex ratio contribute to lower status for women, or perhaps alleviate it? If there were twice as many men as women, would women have more opportunities to choose partners who would support their aspirations?

A male-biased sex ratio could certainly cause "problematic macro-scale consequences for society", but would those consequences be all negative? Population growth can have negative macro-scale consequences, but every country except China has decided that those consequences are not negative enough to justify the "mutual coercion, mutually agreed upon" that Garrett Hardin thought necessary to limit population growth.

Making the hypothetical "boy pill" freely available would hardly qualify as coercion, but a male-biased sex ratio would tend to slow population growth. Could the net benefits of slower population growth (mostly positive) outweigh the net costs of a male-biased sex ratio (probably negative)?

Things get more interesting when we ask who it is that would prefer sons to daughters. In the US, men strongly prefer sons, by 49 to 22%, while women value sons and daughters equally. Conservatives prefer sons, 41 to 25%, while liberals value both equally. If this tendency applies worldwide, then widespread availability of the hypothetical "boy pill" would lead to male-biased sex ratios mainly in more-conservative countries and subcultures where men dominate decision making.

Would a shortage of females slow the population growth of conservatives? That's far from clear. If conservatism were highly heritable, either genetically or culturally, and if conservatives didn't marry outside of their subculture (i.e., if mating were highly assortative), then conservative societies with male-biased sex ratios would tend to grow more slowly. To the extent that conservatism (or a preference for sons) has a genetic basis, those genes would become less common than if the "boy pill" were not available.

Similarly, a "girl pill" could perhaps slow the population growth of any society with a strong preference for girls, assuming both assortative mating and lifelong monogamy.

Sex selection is just one example of the choices that parents may be able to make within the next few decades. Would it matter whether parental choices for the skin pigmentation of their offspring were motivated by their own prejudices, societal prejudices that they personally abhor, or the relative risks of skin cancer versus vitamin-D deficiency at their latitude?


A comment on slower growth of conservative cultures that use a "boy pill": you're assuming equal offspring-per-woman for both conservative and liberal cultures. In conservative cultures women have less freedom and less access to contraception. So the greater number of offspring per woman in a conservative society might offset or more than offset the male-female birth ratio. Just saying.

I agree that conservative societies might still grow faster than liberal ones, but maybe slower than they would have without the boy pill.

Interesting topic, though I think it's hard to do more than speculate given the multitude of factors at play. I agree with your response to Yglesias' first argument (men more valuable than women); as the ratio of M:F increases, F should have more desirable mates to choose from. The polling data are interesting too, but I'd like to see that backed up by quantitative research on the parental investment in boys vs girls (eg parental income spent on education). How commonplace is it for the bride's family to pay for the wedding in the US? Is that changing? I would guess that many of those polled volunteer "no preference" simply because it's expected. Maybe liberals are no different than conservatives in their apparent preference for male offspring, but are less inclined to be honest.


I agree that what people say may not reflect what they would do, and that the discrepancy could differ among groups. Investing more in educating one sex or the other may not be a perfect predictor of preferred sex ratio either, however.

It would be very sad to find out that this "boy pill" is used by a lot of couples. But I'm nobody to judge their choices, they must have a good reason to do it.

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