Fascinating read on how the marriage squeeze, already established in countries like Japan and South Korea, has finally hit a third of the world's population, namely that of China and India. It's not just that sex selection at birth has led to a large gender imbalance in the population. Other factors exacerbate the problem.

Countries with normal sex ratios can experience a marriage squeeze if their fertility rates are falling fast. Fertility is important, because men tend to marry women a few years younger than themselves. In India the average age of marriage for men is 26; for women, it is 22. This means that when a country’s fertility is falling, the cohort of women in their early 20s will be slightly smaller (or will be rising more slowly) than the cohort of men they are most likely to marry—those in their late 20s (this is because a few years will have gone by and the falling fertility rate will have reduced the numbers of those born later). This may not sound like a big deal. But in fact between 2000 and 2010 the number of Indian men aged 25-29 rose by 9.2m. The number of Indian women aged 20-24 (their most likely partners) rose by only 7.6m.

Even if India’s sex ratio at birth were to return to normal and stay there, by 2050 the country would still have 30% more single men hoping to marry than single women. This is explained by a rapid decline in India’s fertility rate. But in China, where fertility has been low for years, the more gradual decline in fertility still means there will be 30% more single men than women in 2055, though the distortion declines after that. A decline in fertility usually benefits developing countries by providing a “demographic dividend” (a bulge of working-age adults compared with the numbers of dependent children or grandparents). But it does have the drawback of amplifying the marriage squeeze.

The problem is further accentuated by a so-called “queuing effect”. The length of a queue is determined by how many people join it, how many leave, and how long queuers are prepared to wait. In the same way, marriage numbers are a result of how many people reach marriageable age (the joiners); how many get married (the leavers) and how long people are willing to wait. In India and China, marriage remains the norm, so men keep trying to tie the knot for years.

Hence, a marriage queue in India and China builds up. At stage one, a cohort of women reaches marriageable age (say, 20-24); they marry among the cohort of men aged 25-29. But there are slightly more men than women, so some members of the male cohort remain on the shelf. Later, two new cohorts reach marriageable age. This time, the men left over from the previous round (who are now in their early thirties) are still looking for wives and compete with the cohort of younger men. The women choose husbands from among this larger group. So after the second round even more men are left on the shelf. And so on. A backlog of unmarried men starts to pile up. Just as you need only a small imbalance between the number of people joining a queue and the number leaving it to produce a long, slow-moving line, so in marriage, a small difference in the adult sex ratio can produce huge numbers of bachelors.

One can't help but conclude that India and China must prepare for an end to universal marriage. Is that so bad? Could both countries start to shift their policies to prepare for a post-universal-marriage society? Are there any countries with economic policies that can cope with declining birth rates?

Perhaps, but it's difficult to imagine a world in which the consequences are anything but a net negative.

There may be positive side effects: a shortage of brides in India is causing dowry prices to fall in some areas, for instance. Overall, though, the impact is likely to be negative. A study by Lena Edlund of Columbia University and others found that in 1988-2004, a one-point rise in the sex ratio in China raised rates of violent crime and theft by six to seven points. The abduction of women for sale as brides is becoming more common. The imbalance is fuelling demand for prostitution.