Ordinary human populations the world over will normally record slightly more births of boys than girls. This world-wide historical ratio has tended to fall in the range of around 103-105 boys for every 100 girls. In contemporary Asia, however, this age-old balance is today coming undone. In China, the imbalance is now around 118-100, and it is even worse if one excludes regions not populated by ethnic Han Chinese.

The reasons are not difficult to surmise: a cultural preference for boys is leading to the selective abortion of female fetuses. As distasteful as that is in itself, this problem of "excess boys" may have far-reaching, even catastrophic implications for social life--and possibly even political affairs.

With China, it became clear with the 1982 census--when a ratio of almost 108 was recorded--that a growing imbalance was not a temporary aberration. In subsequent population counts, China's reported sex ratio at birth rose inexorably: to almost 112 in 1990, then nearly 116 in 1995.

There are, to be sure, reasons to question the accuracy of these numbers: reported birth totals in the 2000 Chinese census, for example, are implausibly low, leaving open the possibility that baby girls are disproportionately undercounted. Hospital data also record a less extreme (albeit still unnatural) trend in sex ratios at birth for those born on their premises.

But the result cannot be dismissed as a statistical artifact. For one thing, there is a striking consistency between the results of successive population counts: the same imbalance that is reported in the 1990 census shows up for five-year-olds in the 1995 census, 10-year-olds in the 2000 census, etc. For another, the reported imbalance for the sex ratio of young children is even higher than that reported for infants. Indeed, in China's 2000 population count, the recorded sex ratio for children aged 1-4 was over 120. Only two provinces in the entire country--the non-Han regions of Tibet and Xinjiang--reported sex ratios within the biologically normal human range. At the other end, three provinces (Hubei, Guangdong and Anhui) tabulated child sex ratios of almost 130--while three others (Hainan, Hunan and Jiangxi) returned with ratios of over 130.

Closer examination suggests these outcomes can be explained as consequences of a collision between three forces. First, a strong and enduring cultural preference for sons. Second, low demographic growth--in China this is partly due to the coercive "one-child policy" still in force in some places. And third, the advent of widespread technology for prenatal sex determination and gender-based abortion.

To judge by the data, Chinese parents are willing to let nature take its course in the sex of their firstborn. They have, however, become increasingly disposed to intervene to assure that a second or third child is a boy. According to the 2000 census, over two-thirds of all "higher order" infants born in the previous year were male.

So the question arises, is Beijing's coercive population-control responsible for these amazing distortions? A tentative answer would be: yes--but not entirely. In other Chinese or Confucian-heritage populations where oppressive population control is not in force--Hong Kong, Taiwan, Singapore, South Korea--unnatural sex ratios at birth also emerged in the 1980s and 1990s. Their imbalances are not as pronounced, however. Indeed, the trend has begun to reverse itself in these other societies. China's rise as yet shows no signs of reversing.

The prospect of steadily diminishing absolute numbers of women of marriageable age sets the stage for an historically unprecedented "marriage squeeze" in the decades ahead. Back-of-the-envelope arithmetic suggest some very large proportion of tomorrow's young Chinese men--certainly over 10 percent, perhaps 15 percent or more--may find themselves essentially "unmarriageable" in the coming decades.

In other places and at other times, significant proportions of the male population lived their lives without marrying. In Western Europe in the pre-industrial and early-industrial periods it was not uncommon for 15 percent to 20 percent of males to remain unmarried. But that pattern was built upon a complex and delicate foundation: a mesh of ethical precepts and arrangements that supported the institution of honorable bachelorhood.

No similar foundations exists in China--where until now the expectation of universal male marriage has prevailed, and where Confucian tradition stresses the son's obligation to marry and honor his ancestors by continuing the family line.

The world has never seen the likes of the bride shortage that will be unfolding in China, so it is difficult to imagine its many reverberations. Some commentators have warned that this "surplus of males" will make for a "deficit of peace" pushing China toward a more martial international posture. That assessment may overstate the actual case for demographically induced risks of international conflict in Asia.

It does not seem wild, however, to propose that the emergence and rise of this phenomenon may occasion an increase in social tensions in China--and perhaps social turbulence as well. Exactly how China's future cohorts of young men are to be socialized with no prospect of settled family life, and no tradition of honorable bachelorhood, is a question that can be asked today, but not answered.

China will be the first great power in Asia to suffer from a 21st century "bride shortage," but it may not be the last. Unsettling trends of a similar nature are already evident in India. Son preference in India remains extremely strong--according to survey results, women venturing a preference for their next birth voted for boys over girls by a ratio of four to one. With declining fertility and the spread of ultrasound, India's sex ratio is already at 108 boys under age 7 for every 100 girls, and it is rising.

For now, we must live with the disturbing possibility that that these particular expressions of "Asian values" will have unpredictable repercussions in these powers for decades to come.

Nicholas Eberstadt holds the Henry Wendt Chair at the American Enterprise Institute.