Analysis of a 2005 census suggests that, in China's under-20 age group, there are almost 33 million more males than there are females.
Therese Hesketh of University College London and her colleagues pin the heightened sex ratio (the number of boys in each age group for every 100 girls) on sex-selected abortions starting with the introduction of low-cost ultrasound in the late 1980s.
The study extrapolates from a survey of nearly 4.8 million people in the under-20 set — covering 1% of this population across all of China's provinces. The authors show the nationwide sex ratio rising from 108 in the late 1980s to 124 in the 2000–2004 period. Male-biased births were highest for rural families who were allowed a second child after having a girl.