The lost boys of Prof. Albert Macovski are upon us. Twenty years ago, the ultrasound scanning machine came into widespread use in Asia. The invention of Macovski, a Stanford University researcher, the device quickly gave pregnant women a cheap and readily available means to determine the sex of their unborn children. The results, by the million, are now coming to maturity in Bangladesh, China, India, and Taiwan. By choosing to give birth to males—and to abort females—millions of Asian parents have propelled the region into an extraordinary experiment in the social effects of gender imbalance.

Back in 1990, Nobel Prize-winning Indian economist Amartya Sen was one of the first to call attention to the phenomenon of an estimated 100 million “missing women” in Asia. Nearly everywhere else, women outnumber men, in Europe by 7 percent, and in North America by 3.4 percent. Concern now is shifting to the boys for whom these missing females might have provided mates as they reach the age that Shakespeare described as nothing but stealing and fighting and “getting of wenches with child.”

Now there are too few wenches. Thanks in large part to the introduction of the ultrasound machine, Mother Nature’s usual preference for about 105 males to 100 females has grown to around 120 male births for every 100 female births in China. The imbalance is even higher in some locales—136 males to 100 females on the island of Hainan, an increasingly prosperous tourist resort, and 135 males to 100 females in central China’s Hubei Province. Similar patterns can be found in Taiwan, with 119 boys to 100 girls; Singapore, 118 boys to 100 girls; South Korea, 112 boys to 100 girls; and parts of India, 120 boys to 100 girls.

China, India, and other nations have outlawed the use of prenatal diagnostic techniques to select the sex of an...