Source: The Los Angeles Times

On a smoggy morning in Lanzhou, a gritty industrial city in China's Gansu province, crowds of young men gather outside a half-built construction site. Dressed in torn jeans and dirty shirts and carrying thermoses of tea, they push toward the exterior fence, jostling for the attention of a site manager who hands out short-term jobs. Most of the men are unmarried and have no families. Finding no work, they drift away from the site and, by midday, congregate at a riverside park, where they trade tea for large bottles of beer, which they gulp down. Many of them soon stumble in circles.

Lanzhou exemplifies a more insidious, possibly more dangerous threat to China's development than financial imbalances, environmental disasters or unemployment: The People's Republic has too many men. Today, roughly 120 boys are born in China for every 100 girls, perhaps the worst gender imbalance in modern human history. Within 15 years, the country may have 30 million men who cannot find wives. That could mean serious trouble.

For centuries, patrilineal Chinese households have preferred male children because men are viewed as better able to support rural families, and boys inherited the land. Some Chinese gender experts, such as Liu Bohong of the All-China Women's Federation, also argue that there is deep-seated male chauvinism in Chinese culture that leads to a preference for boys.

Infanticide often resulted, which sometimes created gender imbalances. But after taking power in 1949, the Communist Party largely stamped out infanticide, and by the early 1980s, China had a relatively normal ratio of male and female babies.

China's one-child policy, launched around the same time and still in force with some minor exceptions, has restored the gender imbalance. Families allowed only one child may be aborting until they have a boy. Ironically, as China has become wealthier, rising incomes have coincided with more, not less, sex selection. Inexpensive modern ultrasounds have enabled parents to learn their child's sex, and if the baby is a girl, they abort.

The policy has another pernicious effect. As American Enterprise Institute scholar Nicholas Eberstadt has documented, it will turn China, by 2030, into a grayer society than the United States. Yet China will still not be as wealthy as the U.S. and will face a tougher time supporting its senior citizens.

Other Asian countries in which a traditional preference for males is abetted by modern medical technology are also becoming bachelor nations. The bride shortage in South Korea is so severe that companies in Seoul advertise Vietnamese wives for desperate Korean men. Taiwan and Pakistan have vastly more male babies than female ones. India faces almost as great a bachelor crisis as China -- by 2020, India may have 28 million men who cannot find wives. As in China, Indian prosperity has made the problem worse: The country's starkest gender imbalances occur in some of its wealthiest states.

As any parent of adolescents knows, boys tend to be happier and calmer when they've found love. In a landmark study, political scientists Andrea M. den Boer and Valerie Hudson also determined that single young men are far more likely to commit violence than their married peers. Even young criminals, they found, often give up crime when they marry and settle down. These findings may be playing out in China, which is experiencing rising crime waves. Cities with the most unbalanced sex ratios have some of the highest crime rates.

The demand for brides also is fueling a different type of crime -- a growing sex trafficking industry in China, one that is sweeping in girls from such neighboring nations as Laos, Myanmar, North Korea and Thailand and poisoning China's image in these countries. Some of these women, such as North Koreans, wind up as virtual slaves in China.

China's surplus males may be developing into a permanent angry underclass capable of being dangerously exploited. As in Lanzhou, unemployed unmarried men dominate China's 150-million-strong pool of migrant labor, and most of them have no prospect of obtaining an education or long-term job. These "surplus males" increasingly congregate in certain areas of cities -- train and bus stations are favorites -- and have begun to form gangs. As China faces a wildfire of protests concerning labor and property rights, as well as other issues -- the number of "mass incidents," or large protests, in the country rose more than 500% between 1994 and 2005 -- companies or local officials have started hiring members of this male underclass as thugs. This has led to more violent confrontations in factories, physical attacks on local activists and journalists, and peasants being forced off land that developers covet.

Worse, as the study by den Boer and Hudson suggests, the Chinese military could recruit from the country's male underclass. Throughout history, one way to use surplus men is to send them abroad to fight wars, and the paramilitary People's Armed Police reportedly has been beefing up its ranks.

Although it would take years for China to achieve gender balance, it is not fated to become a bachelor nation. Beijing is beginning to understand the problem. Once loath to reveal breakdowns in social stability, China's State Population and Family Commission admitted last winter that "the increasing difficulties men face finding wives may lead to social instability." In response, China could relax its one-child policy, which would not only reduce sex-selective abortions but might head off the early graying of China's population.

Better yet, China could create real social-safety nets, ones that provide income for migrant populations and reassure parents that they do not need a son to support them in old age.Right now, despite its supposed commitment to socialism, China has virtually no state welfare system for the elderly.

And if China, as well as other Asian countries, does not care for its bachelors?

The past provides a guide. In the mid-19th century, another period when female infanticide created skewed sex ratios in China, a revolt developed across the countryside, in part because young men were unable to find wives and formed into armed bands. The imperial court crushed the insurgency, called the Nien Rebellion, but it took more than a decade for Beijing to win the battle. Ultimately, these revolts weakened the court and hastened its downfall. It is a lesson China's current rulers surely have not forgotten.

Joshua Kurlantzick is a visiting scholar at the Carnegie Endowment for International Peace and the author of "Charm Offensive: How China's Soft Power Is Transforming the World."