Changes in Income at Macro Level Predict Sex Ratio at Birth in OECD Countries.

Ohto Kanninen,Aleksi Karhula

doi:10.1371/journal.pone.0158943

Ohto Kanninen, Aleksi Karhula

Open Access

https://doi.org/10.1371/journal.pone.0158943

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Abstract

The human sex ratio at birth (SRB) is approximately 107 boys for every 100 girls. SRB was rising until the World War II and has been declining slightly after the 1950s in several industrial countries. Recent studies have shown that SRB varies according to exposure to disasters and socioeconomic conditions. However, it remains unknown whether changes in SRB can be explained by observable macro-level socioeconomic variables across multiple years and countries. Here we show that changes in disposable income at the macro level positively predict SRB in OECD countries. A one standard deviation increase in the change of disposable income is associated with an increase of 1.03 male births per 1000 female births. The relationship is possibly nonlinear and driven by extreme changes. The association varies from country to country being particular strong in Estonia. This is the first evidence to show that economic and social conditions are connected to SRB across countries at the macro level. This calls for further research on the effects of societal conditions on general characteristics at birth.

Highlights

The Trivers-Willard hypothesis [1] ( TWH) predicts that natural selection favors a positive relationship between sex ratio at birth (SRB) and mothers’ ability to invest in offspring
Father’s active role in parental investment and kin interaction between adults complicates the application of TWH to humans, we could expect SRB to vary with environmental factors affecting condition
Most of the observations with large increases in disposable income remaining in the data were from Estonia

Summary

Introduction

The Trivers-Willard hypothesis [1] ( TWH) predicts that natural selection favors a positive relationship between sex ratio at birth (SRB) and mothers’ ability to invest in offspring. This prediction stems from the assumption that a male in good condition at the end of the period of parental investment produces more offspring than a female in similar condition, and a female in bad condition produces more offspring than a male in similar condition. Father’s active role in parental investment and kin interaction between adults complicates the application of TWH to humans, we could expect SRB to vary with environmental factors affecting condition. Theories as to why have been put forth, but the proposed explanations are unable to fully account for the changes [2]

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