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?