Study finds natural variation in sex ratios at birth and sex ratio inflation in 12 countries

An international team of researchers, led by UMass Amherst biostatistician Leontine Alkema and her former Ph.D. student Fengqing Chao, developed a new estimation method for assessing natural variations in the sex ratio at birth (SRB) for all countries in the world.

In the study published Monday, April 15 in Proceedings of the National Academy of Sciences (PNAS), the researchers found natural variation in regional baseline SRBs that differ from the previously held standard baseline male-to-female ratio of 1.05 for most regions.

They also identified 12 countries with strong evidence of sex ratio at birth imbalances, or sex ratio inflation, due to sex-selective abortions and a preference for sons.

“Given that sex ratios at birth are still inflated in some countries and could increase in the future in other countries, the monitoring of the sex ratio at birth and how it compares with expected baseline levels is incredibly important to inform policy and programs when sex ratio inflations are detected,” says Alkema, associate professor in the School of Public Health and Health Sciences.

Alkema adds, “While prior studies have shown differences in the sex ratio at birth, for example based on ethnicity in population subgroups, there is no prior study that we know of that has assessed regional baseline levels of the SRB. When we did the estimation exercise, after excluding data that may have been affected by masculinization of the sex ratio at birth, we found that regional levels differed from the commonly assumed 1.05 in several regions.”

Estimated regional reference levels ranged from 1.031 in sub-Saharan Africa to 1.063 in southeastern Asia and eastern Asia, and 1.067 in Oceania.

Alkema regularly collaborates with United Nations agencies to develop statistical models to assess and interpret demographic and population-level health trends globally. For this study, along with Alkema at UMass Amherst, researchers at the National University of Singapore and the United Nations Population Division compiled a database from vital registries, censuses and surveys with 10,835 observations – 16,602 country-years of information from 202 countries. They developed Bayesian statistical methods to estimate the sex ratio at birth for all countries from 1950 to 2017.

“We found that the SRB imbalance in 12 countries since 1970 is associated with 23.1 million fewer female births than expected,” Alkema says.

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The majority of the missing female births were in China, with 11.9 million, and India, with 10.6 million. The other countries identified with SRB imbalance were Albania, Armenia, Azerbaijan, Georgia, Hong Kong, Republic of Korea, Montenegro, Taiwan, Tunisia and Vietnam.