Sex Ratios at Birth
Across human populations, around 105 boys are born for every 100 girls ā a slight male surplus that compensates for higher male childhood mortality. In several Asian countries the ratio rose well above the natural norm from the 1980s as prenatal sex determination and selective abortion became available. China and India peaked above 115 and have since begun to normalize.
Key insights
The biological baseline is remarkably stable
Across thousands of years of human populations the sex ratio at birth (SRB) hovers between 103 and 107 boys per 100 girls. The small male surplus is a biological adaptation ā male childhood and adolescent mortality runs ~30% higher than female, so a 105:100 birth ratio yields roughly equal numbers at reproductive age. Deviations from this band signal something non-biological.
Skewed ratios followed ultrasound, not policy
China's One Child Policy (1980ā2016) is often blamed for elevated sex ratios. But India, Vietnam, South Korea, Taiwan, the South Caucasus and the West Balkans all show similar 1980sā2000s spikes. The common factor is the availability of cheap prenatal sex determination (ultrasound) interacting with pre-existing son preference. Restricted fertility intensifies the effect but doesn't create it.
Normalisation is well advanced in some countries, not others
South Korea peaked at 117 in 1990 and is back to 105 ā close to biological normal. Taiwan and Vietnam are normalising. China's ratio peaked at 121 in 2008 and is now at 111 ā improving but still elevated. India is at 110, improving slowly. Azerbaijan, Armenia and Albania remain stubbornly above 110 with smaller populations affected.
Sex ratio at birth ā selected countries (2024)
Male births per 100 female births; biological norm ā 105
Key Finding: Most countries cluster around 104ā106. Azerbaijan, China, Vietnam, India sit above 108.
Sex ratio at birth ā trend in selected countries 1980ā2024
Boys per 100 girls
Key Finding: South Korea has returned to biological normal; China's peak (121 in 2008) is unwinding slowly; India lags behind.
Methodology & caveats
Why 105:100?
The biological baseline reflects a slight male-skew in conception combined with higher in-utero male mortality. Across populations without sex selection the figure is consistently 103ā107. Genetic, hormonal and environmental modifiers (paternal age, maternal stress, war-time exposures) shift the number by less than a single unit.
Measurement
SRB is calculated from registered births. Where civil registration is weak (many low-income countries), it is estimated from census-back-projection and household surveys. The UN World Population Prospects compiles harmonized national figures. Real-time SRB is published by national statistical offices; data lags 2ā3 years.
Why ratios are normalising
Multiple factors: rising female education and earnings (raising the perceived value of daughters); declining son-preference in younger cohorts; bans on prenatal sex determination (in India and China, enforced unevenly); and aging populations creating direct demand for daughters as caregivers. Korea's reversal preceded the legal bans; the cultural shift was the more important driver.