New sex ratio study reveals millions of missing female births since 1970

For every 100 babies born biologically female, 105 are born biologically male—nature’s way of balancing out the longer lifespans achieved by women.

This ratio is relied upon by demographers for projections about future population change. However, a study published this week in the Proceedings of the National Academy of Sciences, to catalog global sex ratios at birth, shows that this approach is not entirely accurate.

Fengqing Chao, a public health researcher at the Institute of Policy Studies in Singapore, led a five-year project which studied decades of census data, national survey responses, and birth records to build models that could estimate national sex ratios across time. 

Chao’s team of researchers discovered that in most regions of the world sex ratios diverge significantly from the 100:105 norm, revealing a gap of 23.1 million missing female births across a dozen countries since 1970. The results show how societal values can distort the laws of nature.

During 1970–2017, missing female births occurred mostly in the strongly patriarchal societies of China and India, where sons were chosen over daughters as part of routine reproductive decision-making. The height of sex-ratio inflation in the country peaked in 2005, at about 118 male to 100 female births. 

Wei Huang explores China’s “one-child policy” (OCP) in his IZA World of Labor article: “In the 1970s, after two decades of explicitly encouraging population growth, policymakers in China began enacting a series of measures to curb it,” he writes. “Differing from birth control policies in many other countries, the OCP assigned a compulsory general ‘one-birth’ quota to each couple, though its implementation varied considerably across regions for different ethnicities at different times.” 

The policy lasted for 30 years until it was finally revoked in 2015 after affecting millions of couples. The country’s fertility rate dropped from 2.81 in 1979 to 1.51 in 2000, according to World Bank figures, while the sex ratio at birth (males to females) increased by 0.2 over the course of 25 years as many parents chose to have a male child as their only child.

Huang acknowledges that the OCP curbed a potentially problematic population boom in China, and possibly increased human capital accumulation. But, he writes, “it has also brought with it problems, such as an unbalanced sex ratio, increased crime, and individual dissatisfaction toward the government.”

While China and India are responsible for the majority of the missing female births, the new research shows similar trends in other parts of the world, like Eastern Europe (Albania, Armenia, and Montenegro), and the African nation of Tunisia.

It will take many years for countries like China to return to a biologically normal sex ratio at birth, but the new research at least provides demographers with more accurate estimates.