

A couple watch their baby inside a waiting hall at the Nanjing railway station, capital of Jiangsu province. [Reuters/2006]

In nature, the balance of males and females is maintained by natural selection acting on parents. As Sir Ronald Fisher brilliantly pointed out in 1930, a surplus of one sex will be redressed by selection in favour of rearing the other sex, up to the point where it is no longer the minority. It isn't quite as simple as that. You have to take into account the relative economic costs of rearing one sex rather than the other. If, say, it costs twice as much to rear a son to maturity as a daughter (e.g. because males are bigger than females), the true choice facing a parent is not "Shall I rear a son or a daughter?" but "Shall I rear a son or two daughters?"

So, Fisher concluded, what is equlibrated by natural selection is not the total numbers of sons and daughters born in the population, but the total parental expenditure on sons versus daughters. In practice, this usually amounts to an approximately equal ratio of males to females in the population at the end of the period of parental expenditure.

Note that the word 'decision' doesn't mean conscious decision: we employ the usual 'selfish gene' metaphorical reasoning, in which natural selection favours genes that produce behaviour 'as if' decisions are being made.

Interestingly, Fisher's reasoning remains intact, even in harem-based societies such as those of elephant seals, where a minority of males monopolise the females and the majority of males hang about as disconsolate bachelors. From a parent's point of view, a daughter is a 'safe' choice, likely to yield an average number of grandchildren. A son is a high risk choice. He is most likely to give you no grandchildren at all. But if he does give you grandchildren he'll give you lots. The figures balance out and Fisher's equilibrium still holds.

That's what happens in nature. But what if we are dealing with a human society in which cultural traditions over-ride the genetic imperatives (yet another example, this time not necessarily a benign one, of 'rebelling against the selfish genes'). What if the religion of a country fosters a deep-rooted undervaluing of women? What if there is an ancient culture of despising women, whether for religious or otherwise traditional or economic reasons?

In past centuries such cultures might have fostered selective infanticide of newborn girls. But now, what if scientific culture makes it possible to know the sex of a fetus, say by amniocentesis or ultrasound scanning? There is then an obvious temptation selectively to abort female embryos, which could have far-reaching and probably pernicious social consequences. I'll refrain from gloating over the possibility of Taliban-inspired woman-hating societies going extinct for lack of women.

The Guardian has a report today on 'sex selection of babies', which is described as a 'scourge' of the developing world:

Unnatural Selection by Mara Hvistendahl charts how the trend towards choosing boys over girls, largely through sex-selective abortions, is rapidly spreading across the developing world. While the natural sex ratio at birth is 105 boys born for every 100 girls, in India the figure has risen to 112 boys and in China 121. The Chinese city of Lianyungang recorded an astonishing 163 boys per 100 girls in 2007. The bias towards boys has been estimated to have caused the "disappearance" of 160 million women and girls in Asia alone over the past few decades. The pattern has now spilled over to Azerbaijan, Georgia and Armenia, the Balkans and Albania, where the sex ratio is 115/100. The unnatural skewing towards male populations has become so pronounced in recent decades that Hvistendahl, a writer for Science magazine, says it has given rise to a new "Generation XY". She raises the possibility that with so many surplus men – up to a fifth of men will be single in northwestern India by 2020 – large parts of the world could become like America's wild west, with excess testosterone leading to raised levels of crime and violence.

I'm sure Hvistendahl is right when she says, "Historically, societies in which men substantially outnumber women are not nice places to live", and when she compares it to the American wild west "with excess testosterone leading to raised levels of crime and violence."

But is she right to blame Western science and governments for making sex selection possible? Why do we blame science for offering a method to do bad things? Science is the disinterested search for truth. If you want to do good things, science provides very good methods of doing so. And if you want to do bad things, again science provides the best practical methods. The ability to know the sex of a fetus is an inevitable byproduct of medical benefits such as amniocentesis, ultrasound scanning, and other techniques for the diagnosis of serious problems. Should scientists have refrained from developing useful techniques, for fear of how they might be misused by others?

Even sex selection itself and selective abortion of early embryos is not necessarily a social evil. A society which values girls and boys equally might well include parents who aspire to at least one of each, without having too large a family. We all know families whose birth order goes girl girl girl girl boy stop. And other families of boy boy boy boy girl stop. If sex selection had been an option, wouldn't those families have been smaller: girl boy stop, and boy girl stop? In other words, sex selection, in societies that value sexual equality, could have beneficial effects on curbing overpopulation, and could help provide parents with exactly the family balance they want.

But the general question I want to raise is whether the evils of what Mara Hvistendahl calls the XY generation should be blamed on "western governments and businesses that have exported technology and pro-abortion practices without considering the consequences." Or whether they should be blamed on the cultural and religious practices that despise and discriminate against women in the first place.

Related reading: the book Unnatural Selection by Mara Hvistendahl (PublicAffairs, June 7, 2011).