Researchers from University of Kansas (KU) in the US found those numbers are likely overblown and that a large number of those girls are not missing at all.
"People think 30 million girls are missing from the population. That's the population of California, and they think they are just gone," said John Kennedy, a KU associate professor of political science.
"Most people are using a demographic explanation to say that abortion or infanticide are the reasons they do not show up in the census, and that they do not exist. But we find there is a political explanation," said Kennedy.
In 2015, Chinese state media announced all couples would be allowed to have two children, signalling the end of the controversial 35-year-old policy, but scholars and policymakers are examining how the ban could have lasting social influence in China on everything from elderly care to political stability, they said.
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Kennedy and Shi Yaojiang, from Shaanxi Normal University in China, analysed statistics and found that a combination of late registration and unreported births explain a larger portion of the "missing girls" than previously reported in Chinese sex-ratio-at-birth statistics.
Instead they made tact agreements in allowing families to have extra children in exchange for social stability in their communities.
The cadres, or local governments, would then under-report "out of plan" births that ultimately influenced the national population statistics, researchers said.
"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 where they are implanting policies," he said.
The farmer referred to the middle daughter as "the non-existent one."
Kennedy said since the mid-1980s, villagers could legally have a second child if the firstborn was a girl.
"We noticed that qualitatively when we interviewed villagers and higher and lower level officials everybody had a tacit understanding that yes, millions of girls and some boys, too, were allowed to be unregistered, and then these children appear in the population statistics as older cohorts at junior high school age and marriage age," said Kennedy.