CHINA and India, home to a third of humanity, both face a marriage crisis that will last for generations. As recently as 2010, marriage patterns were normal in the two countries. Now India is revising 500-year-old laws to allow men to marry out of caste, village and state—while in China 50 million men known as guanggun (“bare branches”) look doomed to bachelordom. What has led to this marriage squeeze?

First, millions of women have gone “missing”. A generation ago, a preference for sons and the greater availability of prenatal screening meant first Chinese couples, then Indian ones, started aborting female fetuses and giving birth only to boys. At its extreme, in parts of Asia, more than 120 boys were being born for every 100 girls. The generation with distorted sex ratios at birth is now reaching marriageable age. The result is that men far outnumber women. If China had had a normal sex ratio at birth, its female population in 2010 would have been 720 million. In fact, it was only 655 million, compared with almost 705 million men and boys—50 million surplus husbands.

Fertility rates then accentuate this distortion. When a country’s fertility rate is going down (as it is in India), younger cohorts of people will tend to be smaller than older ones. If men are older than women at marriage, as they usually are, there will be fewer potential brides than husbands because women will have been born later, when fertility is lower. Then there is a queuing effect. Men who cannot find a wife right away go on looking, competing with younger men. As a result, the number of unmarried men piles up, as in a queue. By 2060, there could be more than 160 Chinese and Indian men wanting to marry for every 100 women.

This is a ferocious squeeze in countries where marriage has always been a requirement for being a full member of society. It could be hugely harmful. Almost everywhere, large numbers of single men are associated with high rates of crime and violence. No one really knows how these countries will react.

Update: This blog post has been amended to remove the news peg.