There is an unequal human sex ratio of males to females at birth

All countries with strongly skewed sex ratios at birth in favour of males have cultures that display a preference for sons

The human sex ratio of males to females at birth is not equal. The average natural expected sex ratio at birth is 105 males for every 100 females, with a normal spread of 103 to 107 male births to every 100 female births.

However, in some countries this expected sex ratio at birth is significantly skewed in favour of male births to an extent that cannot be explained by biological factors alone. There is strong evidence across several countries that sex selective abortion is the main pre-natal discrimination underpinning this “missing girls” phenomenon.

Equal numbers of males and females are produced in human conception but the female embryo/fetus is slightly more likely to miscarry during pregnancy then the male, and this explains the expected 105 male births per 100 female births.

All countries with strongly skewed sex ratios at birth in favour of males have cultures that display a strong preference for sons – principally Albania, Armenia, Azerbaijan, China, Georgia, Hong Kong, India, Montenegro, South Korea, Taiwan, Tunisia, Vietnam.


The male to female ratio at birth is particularly high in China and India – in 2017 China had a sex ratio at birth of 115 boys and India a ratio of 111 boys per 100 girls. The ratio in China in 2005 was 119 boys to 100 girls.

Prenatal sex determination followed by sex-selective abortion has skewed sex ratios at birth towards boys in several countries from southeast Europe to southeast Asia since the 1970s. Following the introduction of prenatal diagnosis, including sex determination, in India in the 1970s, most abortions following prenatal diagnosis were girls and over 63 million women are "missing" statistically across India as reported by the Guardian in January 2018.

‘Missing girls’

Although prenatal discrimination is now the main cause of skewed sex ratios at birth, postnatal sex discrimination also occurs and contributes to this phenomenon of missing girls. Born girls can die from neglect or from infanticide.

A recent study published in BMJ Public Health, reports that in the 12 countries listed above skewed sex birth ratios will stabilise and then decline within 20 years, particularly in China and India. Nevertheless, the researchers still forecast a deficit of over 4.7 million girls by 2030 and a further 1 million by 2100.

But if all countries globally at risk of boosting sex ratios at birth in favour of males, including densely populated Nigeria and Pakistan, are included in the calculations, the number of missing girls could increase to 22 million by 2100 (Sub-Saharan Africa would account for 38 per cent of this).

A strong preference for sons stems from a very rigid culture of patrilineality where the social order of families resides with the males and the family lineage passes from father to son. Men are fixed points in the social order, women are moving points – when a daughter marries she moves from her birth family to a new family (Our World in Data).

Having a son in such societies confers social and economic benefits, including: elderly parents often live with their married children, overwhelmingly so with sons; males have greater perceived paid work opportunities than females; when a daughter marries her parents are expected to pay a substantial dowry to the groom or his family.


Some consequences of strongly skewed birth sex ratios towards males, providing a surplus of males in over one third of the world’s population, are easily predicted, but the full economic and social consequences are unknown.

A strong surplus of men automatically means too few women are available to marry all the marriageable men. This causes marriage to be delayed and a significant number of men must forego marriage altogether. And the poorest men are most heavily impacted because in this scenario women have enhanced opportunities to marry upwards. Resentment and social unrest are highly likely, affecting stability and socially sustainable development.

This missing girls phenomenon is a disturbing bioethical problem. Although it is a basic human rights issue it attracts relatively little attention. The authors of the recent BMJ article call for ongoing monitoring of sex ratios at birth in countries with a strong preference for sons and for action to address the factors behind the persistence of sex bias in families and institutions.

William Reville is an emeritus professor of biochemistry at UCC