For the next 20 years, China will have increasingly more men than women of reproductive age, according to the British Medical Journal.
'Nothing can be done now to prevent this,' the researchers said.
Chinese government planners have long known that the desire of couples to have sons was skewing the gender balance of the population.
But the study, by two Chinese university professors and a London researcher, provides some of the first hard data on the extent of the disparity and the factors contributing to it.
The trend towards more male than female children intensified steadily after 1986, they said, as ultrasound tests and abortions became more available.
'Sex-selective abortion accounts for almost all the excess males,' the paper said.
Professor Wei Xingzhu from Zhejiang Normal University, Prof Li Lu from Zhejiang University and University College London lecturer Therese Hesketh analysed data from a 2005 census.
In 2005, they found that births of boys in China exceeded births of girls by more than 1.1 million.
There were 120 boys born for every 100 girls. The finding, they wrote, was perhaps unsurprising in the light of China's one-child policy. The disparity was widest among children of ages one to four, a sign that the greatest imbalances among the adult population lie ahead, according to the researchers.
They also found more distortion in provinces that allow rural couples to have a second child if the first one is a girl, or in cases of hardship. Those couples were determined to ensure that they had at least one son, the researchers noted.
Among second-born children, there were 143 boys for 100 girls, the data showed.
The Chinese government is openly concerned 'about the consequences of large numbers of excess men for social stability and security', the researchers noted.
The researchers said that enforcing the ban against sex-selective abortions could normalise the sex ratio in the future. New York Times
