Twenty years ago, the ultrasound scanning machine came into widespread use in Asia. The invention of Dr. 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 millions, 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 unborn child. But bribery and human ingenuity have made it easy for prospective parents to skirt the law; a suitably compensated ultrasound technician need only smile or frown at the expectant mother.

Many of the excess boys will be poor and rootless, a lumpenproletariat without the consolations of sexual partners and family. Prostitution, sex tourism and homosexuality may ease their immediate urges, but Asian societies are witnessing far more dramatic solutions.

Women now risk being kidnapped and forced into prostitution and wedlock. Mass sexual frustration is adding a potent ingredient to an increasingly volatile regional cocktail of problems that include surging economic growth, urbanization, drug abuse and environmental degradation.

Understanding the effect of the testosterone overload may be most important in China, the rising Asian superpower. Prompted by expert warnings, the Chinese authorities are already groping for answers.

In 2004, President Hu Jintao asked the country's senior demographers to study whether the country's one-child policy - which sharply accentuates the preference for males - should be revised. Beijing expects it may have as many as 40 million frustrated bachelors by 2020. The regime, always nervous about social control, fears that they might generate social and political instability.

Brigham Young University political scientist Valerie Hudson - the leading scholar on the phenomenon of male overpopulation in Asia - sees historical evidence for these concerns. In 19th-century northern China, drought, famine and locust invasions apparently provoked a rash of female infanticide. According to Dr. Hudson, the region reached a ratio of 129 men to every 100 women. Roving young men organized themselves into bandit gangs, built forts and eventually came to rule an area of some 6 million people in what was known as the Nien Rebellion.

The state's response to crime and social unrest could prove to be a defining factor for China's political future. The CIA asked Dr. Hudson to discuss her dramatic suggestion that "in 2020, it may seem to China that it would be worth it to have a very bloody battle in which a lot of their young men could die in some glorious cause."

Other experts aren't so alarmed. Military observers point out that China is moving from a conscription army to a leaner, professional military. And other scholars contend that China's population is now aging so fast that the elderly may well balance the surge of frustrated young males to form a calmer and more peaceful nation.

It would be reassuring to assume that China's economic growth will itself solve the problem, as prosperity removes the traditional economic incentives for poor peasants to have sons who can work the land rather than daughters who might require costly dowries. But the numbers don't support that theory. Indeed, the steepest imbalance between male and female infants is found in more prosperous regions, such as Hainan Island. And census data from India suggest that slum-dwellers and the very poor tend to raise a higher proportion of female children than more prosperous families.

A Beijing power struggle between cautious old technocrats and aggressive young nationalists may be decided by mobs of rootless young men, demanding uniforms, rifles and a chance to liberate Taiwan. More likely, the organized crime networks that traffic in women will shift their deliveries toward Asia and build a brothel culture large enough to satisfy millions of sexually frustrated young men.

Whatever the outcome, the consequences of Albert Macovski's useful invention will be with us for some time. When they called him "the most inventive person at Stanford," they didn't know the half of it.

Martin Walker is editor of United Press International and senior fellow of the World Policy Institute at the New School University in New York. This essay appears in the Spring 2006 issue of Foreign Policy magazine.

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