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Projecting Sex Imbalances at Birth At Original research BMJ Glob Health: first published as 10.1136/bmjgh-2021-005516 on 2 August 2021. Downloaded from Projecting sex imbalances at birth at global, regional and national levels from 2021 to 2100: scenario- based Bayesian probabilistic projections of the sex ratio at birth and missing female births based on 3.26 billion birth records 1 2 3 Fengqing Chao , Patrick Gerland, Alex Richard Cook, Christophe Z Guilmoto,4 Leontine Alkema5 To cite: Chao F, Gerland P, ABSTRACT Key questions Cook AR, et al. Projecting sex Introduction Skewed levels of the sex ratio at birth imbalances at birth at global, (SRB) due to sex- selective abortions have been observed regional and national levels What is already known? in several countries since the 1970s. They will lead to from 2021 to 2100: scenario- ► No prior study has constructed scenario- based pro- long- term sex imbalances in more than one- third of the based Bayesian probabilistic jections for the sex ratio at birth and missing female projections of the sex ratio at world’s population with yet unknown social and economic births for all countries. birth and missing female births impacts on affected countries. Understanding the potential ► Approaches used for sex ratio at birth projections based on 3.26 billion birth evolution of sex imbalances at birth is therefore essential include the assumption of constant country- specific records. BMJ Global Health for anticipating and planning for changing sex structures levels from 2017 to 2100 in the Global Burden of 2021;6:e005516. doi:10.1136/ across the world. bmjgh-2021-005516 Disease study and projections based on expert Methods We produced probabilistic SRB projections opinion and linear extrapolation by the UN World from 2021 to 2100 based on different scenarios of sex Population Prospects and the US Census Bureau. Handling editor Sanni Yaya ratio transition and assessed their implications in terms ► Additional online of missing female births at global, regional and national What are the new findings? supplemental material is ► We provided projections of the sex ratio at birth and levels. Based on a comprehensive SRB database with http://gh.bmj.com/ published online only. To view, 3.26 billion birth records, we project the skewed SRB and missing female births based on two scenarios for all please visit the journal online missing female births with a Bayesian hierarchical time countries till 2100. (http:// dx. doi. org/ 10. 1136/ series mixture model. The SRB projections under reference ► Our projections were based on Bayesian hierarchi- bmjgh- 2021- 005516). scenario S1 assumed SRB transitions only for countries cal time series mixture models that were fitted to with strong statistical evidence of SRB inflation, and the an extensive database on national sex ratio at birth. The hierarchical structure of the model allowed for Received 25 February 2021 more extreme scenario S2 assumed a sex ratio transition sharing information on sex ratio transitions across on October 6, 2021 by guest. Protected copyright. Accepted 19 June 2021 for countries at risk of SRB inflation but with no or limited countries and for projecting the potential future in- evidence of ongoing inflation. flation process under different scenarios. Results Under scenario S1, we projected 5.7 (95% Our study provided insights into future changes in uncertainty interval (1.2; 15.3)) million additional missing ► the sex ratio at birth as well as the uncertainty asso- female births to occur by 2100. Countries affected will ciated with these changes. be those already affected in the past by imbalanced SRB, such as China and India. If all countries at risk of SRB inflation experience a sex ratio transition as in scenario S2, INTRODUCTION © Author(s) (or their the projected missing female births increase to 22.1 (12.2; Over the last 40 years, prenatal gender- biased employer(s)) 2021. Re- use 39.8) million with a sizeable contribution of sub- Saharan sex selection has become the most visible permitted under CC BY-NC. No Africa. consequence of son preference. Along with commercial re- use. See rights Conclusion The scenario-based projections provide and permissions. Published by child marriage and female genital mutila- important illustrations of the potential burden of future BMJ. tion, sex selection is one of the key harmful prenatal sex discrimination and the need to monitor For numbered affiliations see SRBs in countries with son preference. Policy planning practices defined by the United Nations end of article. (UN) and targeted under the Sustainable will be needed in the years to come to minimise future 1 Correspondence to prenatal sex discrimination and its impact on social Development Goals (SDGs). Sex- selective Dr Fengqing Chao; structures. abortions, the main mechanism behind sex fengqing. chao@ kaust. edu. sa selection, have been observed across a range Chao F, et al. BMJ Global Health 2021;6:e005516. doi:10.1136/bmjgh-2021-005516 1 BMJ Global Health inflation across the world, in already affected countries BMJ Glob Health: first published as 10.1136/bmjgh-2021-005516 on 2 August 2021. Downloaded from Key questions and in those at risk. The main challenge is to understand What do the new findings imply? whether birth masculinity will stay indefinitely skewed in ► The scenario-based projections of sex ratio at births showed that countries affected by sex- selective abortions and whether the sex ratio at birth is most likely to stabilise and decline within new countries may be affected in the future. less than 20 years in countries currently affected by sex imbalances No rigorous attempt has been made so far to outline at birth. and project the future evolution of the SRB imbalances ► However, there will be a deficit of more than 4.7 million female in the world and to estimate its implications on the poten- births between 2021 and 2030 under our conservative scenario. tial deficit of future female births. The most recent set ► A less optimistic scenario envisages a rise in the sex ratio in oth- of world population projections from the Global Burden er countries with strong prevalence of son preference, including in of Disease Study assumes for instance birth masculinity populous countries such as Nigeria or Pakistan. to remain constant through 2100 at their 2017 levels, an unlikely feature in view of the rapid changes in SRB levels observed in countries with skewed SRBs.23 The of various countries from Southeast Europe to South population projections published every 2 years by the and East Asia.2–11 They lead to a hike in the sex ratio at UN derives SRB trends for the next decades from expert birth (SRB; namely the ratio of male to female live births) opinion and linear extrapolation.24 The International above its natural (biological) level and to the emergence Data Base developed by the US census Bureau similarly of a surplus of male births. Along with excess female projects the SRB by linear extrapolation.25 mortality, this SRB inflation is now a major contributor The absence of evidence-based SRB projections stems to the ‘missing women’, a concept first described by mainly from the unique character of the human- made Sen to refer to a population with much fewer females SRB imbalances observed over the last 40 years and the than males.12 According to recent studies, prenatal sex corresponding lack of historical references from which to selection accounts for about half of the recent deficit of infer its future evolution. A more technical factor relates females in the world during the previous decades.13 14 A to the apparently non- linear trends observed in several male- biased sex structure in a society could lead to demo- countries. Mortality and fertility tend to follow mono- graphic issues such as marriage squeeze with lack of tonic and near- linear patterns such as long-term declines marriageable females.15 16 Fewer- than- expected females that are easier to model and project. In contrast, the SRB in a population could result in elevated levels of antisocial trends observed so far appear to follow a more complex behaviour and violence, and may ultimately affect long- and non- monotonic cycle, characterised by a three- stage term stability and social sustainable development.17–20 transformation: an increase, stabilisation and an ultimate Data limitations necessitate the use of models for decrease of the SRB, referred to as the sex ratio transi- estimating SRB levels and trends.13 Population statis- tion.26 The SRB transition process has been observed in tics derived from birth registration, census figures and several countries in the past decades.13 The curved bell- demographic surveys have contributed to estimates of shaped pattern cannot be modelled by a simple linear http://gh.bmj.com/ the SRB in different countries and of its inflation due regression model and requires additional hypotheses or to prenatal sex selection.21 However, SRB series in many covariates to recreate. However, the experience of coun- affected countries are spotty—for lack of reliable civil tries already affected by SRB inflation can now help us registration—and SRB rates must therefore often be delineate the expected patterns of SRB transformations in re- estimated from a variety of sources. In prior work, we relation to its original drivers. We can use for this purpose gathered a comprehensive database of SRB measure- the existing database of country- level SRB measurements on October 6, 2021 by guest. Protected copyright. ments and provided a new set of SRB estimates for all from 1970 to 2017 and an extended modelling approach countries during 1950–2017 along with estimates of the to develop a set of projection scenarios. This model will natural SRB level in the absence of deliberate sex selec- also make possible the estimation of the probability of a tion.13 The total number of female births missing over future SRB rise and subsequent decline in countries that 1970–2017 due to prenatal sex selection was estimated at have so far not been affected by SRB imbalances in spite 45.0 million with a 95% uncertainty interval (34.4; 54.8) of pronounced son preference.
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