This year, millions of young men in China and India will reach their 19th birthday with little prospect of finding a wife. It's not that young, single women aren't available - it's that they don't exist in the same numbers.
Many of these men caught in a marriage squeeze are the first of a generation, born in the mid-'80s, whose mothers used the new technology of ultrasound to determine if their fetuses were female or male. More likely than not, if the fetus was a female, it was aborted.
In many Asian nations, an age-old preference for sons, combined with new prenatal methods of knowing the sex of a fetus, have produced a lopsided gender imbalance that's only now becoming a potential social problem - and not just for the masses of lonely bachelors.
A new book by two Western scholars warns that the widespread practice of sex-selective abortion - despite being outlawed years ago in India and China - could add to societal instability and violent crime, possibly pushing governments to take drastic, antidemocratic steps.
That's because the skewed sex ratios in these societies are staggering. In its 2000 census, China found the proportion of boys through age 4 was more than 120 to every 100 girls at those ages. In India, the pattern is about the same, with reports of some villages having no girls. In most societies, the normal sex ratio for this age group is only 105 to 100 or less.
The book, "Bare Branches: The Security Implications of Asia's Surplus Male Population," (M.I.T. Press), by Valerie Hudson and Andrea Den Boer, connects the dots of a huge demographic trend that carries international implications. ("Bare branches" comes from a Chinese phrase for adult offspring who don't bear children, like empty fruit trees.)
Policymakers should take note: China, India, and other nations that can't stop this practice might see great social upheavals, such as mass migration of young males or the widespread kidnapping of women.
"The security logic of high sex-ratio societies predisposes nations to see some utility in interstate conflict," the authors write.
Conservative estimates put the number of excess men in China at 30 million by 2020, and 28 million for India. Or, to put it another way, some experts say the world's two most populous nations, which have almost 40 percent of humanity between them, are already "missing" 100 million women.
This gender deficit is clearly sex discrimination on a mass feticidal scale.
This modern-day practice of terminating pregnancies because a fetus is the "wrong" sex is an extension of an old practice in traditional societies to kill unwanted baby girls, or neglect them until they die. (In today's China, many girls are simply put up for foreign adoption.)
Boys have long been preferred by rural families because they are reliable help in farming and old age, and also seen as carrying on the ancestral line.
In India, the birth of a girl can mean the heavy burden of paying a large dowry to her future in-laws. "Raising a girl is like watering another family's garden," goes an old Indian saying.
As both China and India have discovered, passing laws against such practices isn't enough. Governments are struggling because couples eagerly seek sex-selection and officials are unable to stop doctors in small clinics who are all too willing to ignore the laws for a small fee. Mobile ultrasound units, too, can easily move from village to village.
Raising the status of women is the ultimate solution to reversing the attitude that girls aren't the equal of boys. That will require years of leadership to counter deep cultural influences. In China, for instance, the State Family Planning and Population Commission recently began a pilot initiative called "Care for Girls." It includes making sure girls get an education and easing the worries of families about old age by offering insurance.
In fact, the more these nations have adequate social security, the more a parental preference for sons should diminish.
This historic shift in gender demography can be curtailed if leaders act boldly. In the meantime, the world must learn to deal with the rising ranks of rootless men.