In greater numbers than ever, China's villagers are using inexpensive prenatal scans and then abortion to prevent the birth of unwanted daughters and to ensure that they will bear a son, recent studies and census data show.

Through the last decade, a time of rapid economic growth, the gap between male and female births only widened, giving China the largest gender disparity among newborns of any country in the world. In pockets of the countryside the imbalance is staggering, with births of as many as 144 boys recorded for every 100 girls.

Decades of Communist rule and recent pell-mell development, far from uprooting the male bias in the Chinese countryside, have yielded new pressures to have sons.

...in much of rural China, daughters move away to their husband's family at marriage, and the government no longer provides even a pretense of old-age or medical care.

This compares with the international average of about 106 boys to 100 girls.

Social scientists also speculate about the disruptive effects another decade or two from now, when there will be tens of millions of excess men in a country with a population of more than 1.3 billion, unable to marry and likely to be concentrated on the bottom rungs of society.

In the biggest cities, the excess of male births is surprisingly low despite the existence over the last two decades of a strict one-child policy. In part, the low urban disparity reflects the more effective enforcement of laws against prenatal gender determination.

But it also reflects a greater acceptance of female children, not least because many urban couples can expect that daughters will be more apt to assist them when they grow old.

Valerie Hudson, a professor at Brigham Young University in the United States is cited in the Economist on the potential consequences:

societies with large numbers of unmarried males tend to experience more crime, unrest and violence. While acknowledging that sex imbalance is only one of many factors influencing levels of violence, Miss Hudson points out that the 30m unhappy unmarried men China is likely to have by 2020 could become "kindling for forces of political revolution at home". There could also be an impact outside China, she says. The government may decide to use the surplus men as a weapon for military adventurism and "actively desire to see them give their lives in pursuit of a national interest".