Jack Loh / flickr
It's what happens when cultural norms and state policies combine in places like China and India to make the birthrates of boys greatly outpace those of girls.
Then, when those boys come of age, there aren't enough would-be brides to go around.
This is a huge problem for at least two reasons:
Historically, heterosexual marriage is seen as a necessary part of being included in society in these two countries
As the New York Times reports, having "a shortage of marriageable women, results in higher rates of crime, including rape, committed by young unmarried men."
The squeeze stems largely from years of sex-selective abortions, first in China as a result of the one-child policy and, in India, as a result of the increasing availability of prenatal screening and a preference for boys. Where there isn't sex-selective abortion, infant girls are killed.
Today, the imbalance is pretty profound:
According to a 2013 BBC report, there are 20 million more men in China then there are women.
According to that same report, more than 33% of men aged 25 to 29 in China are unmarried, and about 20% of women are unwed.
According to 2011 census data, there are 37 million more men than women in India.
And as data collected by the Economist shows, the imbalance is getting even more extreme:
From 2010 to 2015, China's birth ratio was 116 boys to 100 girls, India's was 111 to 100. The natural rate is 105 to 100.
If births rates were normal, China would have 66 million more girls born in 2010. For India, it would have been 43 million.
In 2050, India will have an estimated 30% more men looking to marry than single women. In 2055, it will be the same.
In China in 2050, there will be an estimated 186 single men looking to marry for every 100 single women. For India in 2060, there will be 191 single men looking to marry for every 100 women.
Because of this imbalance, a "marriage queue" starts to develop.
Again, the Economist explains:
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.
Since men in India and China tend to marry women who are younger than them or have less education, professional, educated women in Mumbai and Shanghai have more trouble finding a husband - provided that they're interested in such a thing. That creates an increase in the number of single women age 27 or older, who in China are unhelpfully called sheng nu, or leftover women.
The increase of women's status in society pushes the trend even further.
As can be seen across the globe, the more educated and higher status women become, the more likely they are to put off marriage or avoid getting hitched at all. In the United States, the average age of marriage for women is 27 and 29 for men, both all-time highs. In Europe, the average age of marriage for women is even higher: 32 years old in Sweden, 31 in France and Ireland, and 30 in Italy. In the richest parts of Asia, the average age of marriage is going up, too: In Japan, South Korea, and Hong Kong, the average age of first marriage for women is 29 to 30.
With all these trends combining, marriage in China and India is becoming ever-so-slowly more like marriage in the United States - optional.