Academics often talk about between 30 and 60 million "missing girls" in China, apparently killed in the womb or just after birth, thanks to a combination of preference for sons and the country's decades under a repressive one-child policy.

Now researchers in the United States and China think they might have found many - or even most - of them, and argue they might not have been killed after all.

John Kennedy of the University of Kansas and Shi Yaojiang of Shaanxi Normal University have released a study claiming that the births of many of the girls may, in fact, simply not have been registered.

"People think 30 million girls are missing from the population. That's the population of California, and they think they're just gone," said Kennedy, an associate professor of political science said. "Most people are using a demographic explanation to say that abortion or infanticide are the reasons they don't show up in the census and that they don't exist. But we find there is a political explanation." Local officials, they argue, were complicit in the concealment to retain support from villagers, and maintain social stability.


"There is no coordination between cadres saying 'we're all in agreement,"' Kennedy said. "Actually it's just very local. The people who are implementing these policies work for the government in a sense. They are officials, but they are also villagers, and they have to live in the village."

China abandoned the one-child policy this year after more than three decades, allowing everyone to have two children.

The pair stumbled on their theory when interviewing a villager in China's Shaanxi province in 1996. The man had two daughters and a son, and referred to the younger daughter as "the nonexistent one". Since the mid-1980s, villagers could legally have a second child if the firstborn was a girl.

After more interviews showed the practice to be widespread, the researchers then compared the number of the number of children born in 1990 with the number of 20-year-old Chinese men and women in 2010. They discovered four million additional people, and of those there were about 1 million more women than men. "If we go over a course of 25 years, it's possible there are about 25 million women in the statistics that weren't there at birth," Kennedy said.

The 2010 Chinese census found the sex ratio at birth was 118 males for every 100 females. Globally the average is about 105 males to females.

The findings are published this month in China Quarterly.