The prospect of millions of men forced to go solo threatens major social and political problems for the tightly controlled country of 1.3 billion people, the most populous nation on the planet, experts told the conference on Wednesday.

China, like its neighbours Taiwan and South Korea, has paid the price for a rapid decline in fertility combined with a preference for male children, according to University of Texas researchers Dudley Poston and Karen Glover.

Since the 1980s, modern medical techniques which can determine the sex of a baby before birth, have led to high rates of abortion for female babies, according to the findings.

"From the year 2000 and continuing until 2020, there will be many extra boys of marriage age seeking females to marry, who will be unsuccessful in their courtship pursuits," the researchers said.

Bare branches

During China's baby boom of the 1960s the fertility rate peaked at 7.5 children per woman - compared to 3.7 at the height of the US baby boom in the 1950s - but plummeted following the introduction of the one-child policy in 1979, to 1.7 children per woman in 2001.

Despite the assertion by former Chinese leader Mao Zedong that "women hold up half the sky", the one-child policy led to massive sex selection in favour of boys from the 1980s.

As the ratio reached 120 male babies for every 100 female, the excess boys became known in Chinese as the guang guan or "bare branches".

Prone to crime

According to the research, men who do not marry are more prone to crime than if they were married, raising fears of social and political instability.

To alleviate the problems of an excess male population, Chinese authorities in the past have recruited single men into high-risk law enforcement, military and large scale public works projects which have above average mortality rates.

"China could well turn to an authoritarian form of government. In such a scenario the country's slow progress toward democracy could be stalled if not halted," the researchers said.

Massive female immigration to China to counterbalance the problem was unlikely, they added.

Bachelor ghettos

Instead they suggested the most probable scenario was that excess single men "will settle in bachelor ghettos in Beijing, Shanghai and Guangzhou and Tianjin where commercial sex outlets would be prevalent".

China's neighbours Taiwan and South Korea have also been experiencing abnormally high ratios of male-born children.

All three countries have Confucian patriarchal society structures where the male line inherits the wealth and wives are absorbed into the husband's family.

Previous gender imbalances in China have led to major political upheaval.

In the 19th century, the Nien rebellion in northern Shandong province was in part blamed on an excess male population, estimated at 100,000, that was caused by an earlier famine and widespread female infanticide, the researchers said.

It took the ruling Qing dynasty 17 years to overthrow the rebels who ruled the region of six million people.