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      Systematic assessment of the sex ratio at birth for all countries and estimation of national imbalances and regional reference levels

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          This study provides information on sex ratio at birth (SRB) reference levels and SRB imbalance. Using a comprehensive database and a Bayesian estimation model, we estimate that SRB reference levels are significantly different from the commonly assumed historical norm of 1.05 for most regions. We identify 12 countries with strong statistical evidence of SRB imbalance: Albania, Armenia, Azerbaijan, China, Georgia, Hong Kong (SAR of China), India, Republic of Korea, Montenegro, Taiwan (Province of China), Tunisia, and Vietnam.


          The sex ratio at birth (SRB; ratio of male to female live births) imbalance in parts of the world over the past few decades is a direct consequence of sex-selective abortion, driven by the coexistence of son preference, readily available technology of prenatal sex determination, and fertility decline. Estimation of the degree of SRB imbalance is complicated because of unknown SRB reference levels and because of the uncertainty associated with SRB observations. There are needs for reproducible methods to construct SRB estimates with uncertainty, and to assess SRB inflation due to sex-selective abortion. We compile an extensive database from vital registration systems, censuses and surveys with 10,835 observations, and 16,602 country-years of information from 202 countries. We develop Bayesian methods for SRB estimation for all countries from 1950 to 2017. We model the SRB regional and national reference levels, the fluctuation around national reference levels, and the inflation. The estimated regional reference levels range from 1.031 (95% uncertainty interval [1.027; 1.036]) in sub-Saharan Africa to 1.063 [1.055; 1.072] in southeastern Asia, 1.063 [1.054; 1.072] in eastern Asia, and 1.067 [1.058; 1.077] in Oceania. We identify 12 countries with strong statistical evidence of SRB imbalance during 1970–2017, resulting in 23.1 [19.0; 28.3] million missing female births globally. The majority of those missing female births are in China, with 11.9 [8.5; 15.8] million, and in India, with 10.6 [8.0; 13.6] million.

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          Most cited references 63

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          Natural Selection of Parental Ability to Vary the Sex Ratio of Offspring

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            Natural selection of parental ability to vary the sex ratio of offspring.

            Theory and data suggest that a male in good condition at the end of the period of parental investment is expected to outreproduce a sister in similar condition, while she is expected to outreproduce him if both are in poor condition. Accordingly, natural selection should favor parental ability to adjust the sex ratio of offspring produced according to parental ability to invest. Data from mammals support the model: As maternal condition declines, the adult female tends to produce a lower ratio of males to females.
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              Why is Son preference so persistent in East and South Asia? a cross-country study of China, India and the Republic of Korea


                Author and article information

                Proc Natl Acad Sci U S A
                Proc. Natl. Acad. Sci. U.S.A
                Proceedings of the National Academy of Sciences of the United States of America
                National Academy of Sciences
                7 May 2019
                15 April 2019
                15 April 2019
                : 116
                : 19
                : 9303-9311
                aInstitute of Policy Studies, Lee Kuan Yew School of Public Policy, National University of Singapore , Singapore 259599;
                bPopulation Estimates and Projections Section, United Nations Population Division, Department of Economic and Social Affairs, United Nations , New York, NY 10017;
                cSaw Swee Hock School of Public Health, National University of Singapore and National University Health System , Singapore 117549;
                dDepartment of Biostatistics and Epidemiology, School of Public Health and Health Sciences, University of Massachusetts , Amherst, MA 01003-9304
                Author notes
                1To whom correspondence should be addressed. Email: chao.fengqing@ 123456gmail.com .

                Edited by Jakub Bijak, University of Southampton, Southampton, United Kingdom, and accepted by Editorial Board Member Adrian E. Raftery March 11, 2019 (received for review July 23, 2018)

                Author contributions: F.C. and L.A. designed research; F.C. and L.A. performed research; F.C., P.G., A.R.C., and L.A. analyzed data; F.C., P.G., A.R.C., and L.A. wrote the paper; and F.C. and P.G. compiled the database.

                Copyright © 2019 the Author(s). Published by PNAS.

                This open access article is distributed under Creative Commons Attribution-NonCommercial-NoDerivatives License 4.0 (CC BY-NC-ND).

                Page count
                Pages: 9
                Funded by: National University of Singapore (NUS) 501100001352
                Award ID: WBS R-608-000-125-646
                Award Recipient : Fengqing Chao Award Recipient : Alex R Cook Award Recipient : Leontine Alkema
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