Sex ratio at birth throughout the 20th and 21st centuries, and «missing women»

Sex ratio at birth or «secondary sex ratio» (SSR) is linked to preferences for family size and distribution by gender, which establish the essential basis of population structure. The universal bias of SSR with male predominance, which has characterised our species, was initially interpreted as an unequal proportion of conceptions by gender, with a majority of male foetuses. However, later research showed a balanced proportion of sexes at the moment of fertilisation (1:1), on which differential mortality by gender acts throughout pregnancy, and which is slightly higher for female foetuses resulting on the systematic predominance of male births. Most human populations (including Spain) have shown considerable constancy in their SSR of around 103-108 male babies for every 100 female ones, with certain variations between populations by age and maternal origin, sub-Saharan populations at the lower limit and Asian populations at the higher.

However, several countries (some with large populations) have experienced a significant increase in male births as a result of cultural selection against female ones, which is carried out through abortions and infanticide. Of the 11 countries with the highest SSR imbalance, six are in Asia (China, Hong Kong, India, South Korea, Taiwan and Vietnam) and five in Central Eurasia (Albania, Armenia, Azerbaijan, Georgia and Montenegro).

In 1990 the economist Amartya Sen (Nobel Prize in 1998 and Premio Princesa de Asturias in 2021) established the idea of «missing women» to refer to the additional number of women who might be alive were it not for the following situations: a) an increase in the selective abortions according to gender, b) female infanticide and c) discrimination of girls, which determines a higher rate of mortality than among boys. The growing number of countries with highly biased SSR in favour of boys (even higher than 109 boys per 100 girls) was quite clear at the beginning of the 21st century and set the alarm bells ringing among politicians and experts. The estimated number of missing women was 136 million in 2015, of which 80% correspond to China and India (68 and 45 million, respectively).

The development of techniques to get foetuses of the gender required (selective abortions and implants of embryos whose gender was known) and differential legalisation between countries determined important geographical bias towards male births from the early 1990s, and this reached a maximum around the year 2000. Since then, the rate has dropped continuously. Predictions about the serious consequences on population arising from this bias in SSR led to a change in behaviour between 2000 and 2010 promoted through campaigns to reduce excess male births as reflected in the SSR of all these populations. The last decade has seen a reduction in bias in the sex ratio at birth (so favourable to males) in the two populations mentioned, although China in 2019 still had the most biased SSR in the world (112/100). However, the protraction over time of this selective process against female births in countries with large populations seriously affects, on a global level, the sex ratio at the reproductive ages and significantly reduces the effective size of populations affected, as it prevents a balanced partnership formation among reproductive adults.

The final consequence of the bias towards masculinisation of births which was achieved and maintained by these countries now translates into important imbalances in the formation of relationships between couples of reproductive ages, creating both social and demographic policy problems, which were predicted early on by some experts and are currently being confirmed by the political authorities in those countries, which have to cope with serious problems of social stability. The problem is particularly serious in China, where the only child policy, in force until 2013 (except among certain ethnic minorities who were allowed up to three children), together with a preference for males, determined an increase in female abortions (with a maximum SSR of 117 in 2007) and a biased sex ratio at the reproductive age. The 2021 census shows a surplus of 35 million men compared to the number of women.

This data should make both politicians and citizens in general reflect on the serious consequences arising from political interference in and manipulation of essential aspects of our species’ biology, such as sex ratio at birth.

Cristina Bernis, Professor of Physical Anthropology (Universidad Autónoma de Madrid, Spain), is co-director of the Virtual Museum of Human Ecology. 

A documented database is available about recent temporary change in sex ratio at birth (and later ages) in Our World in Data. On this subject, we recommend the paper by Bongaarts J and Guilmoto CZ (2015). How many more missing women? Excess female mortality and prenatal sex selection, 1970–2050. Population and Development Review, 41(2), 241-269.