Why Is Infant Mortality Higher in Boys Than in Girls? A New Hypothesis Based on Preconception Environment and Evidence From a Large Sample of Twins

Abstract

Infant mortality is higher in boys than girls in most parts of the world. This has been explained by sex differences in genetic and biological makeup, with boys being biologically weaker and more susceptible to diseases and premature death. At the same time, recent studies have found that numerous preconception or prenatal environmental factors affect the probability of a baby being conceived male or female. I propose that these environmental factors also explain sex differences in mortality. I contribute a new methodology of distinguishing between child biology and preconception environment by comparing male-female differences in mortality across opposite-sex twins, same-sex twins, and all twins. Using a large sample of twins from sub-Saharan Africa, I find that both preconception environment and child biology increase the mortality of male infants, but the effect of biology is substantially smaller than the literature suggests. I also estimate the interacting effects of biology with some intrauterine and external environmental factors, including birth order within a twin pair, social status, and climate. I find that a twin is more likely to be male if he is the firstborn, born to an educated mother, or born in certain climatic conditions. Male firstborns are more likely to survive than female firstborns, but only during the neonatal period. Finally, mortality is not affected by the interactions between biology and climate or between biology and social status.

Influence des facteurs environnementaux et démographiques dans la cooccurrence de la diarrhée et de la fièvre chez les enfants dans cinq quartiers périphériques de Ouagadougou (Burkina Faso)

“…Les effets de l'environnement, par exemple, pourraient varier systématiquement selon l'âge ou le sexe (Dabiré, 2002 ;LeGrand et Mbacké, 1995 ;Pongou, 2013).…”

Mortality audit in the paediatrics department of the University of Uyo teaching hospital, Uyo, Nigeria

“…[14][15][16][17][18][19][20] In most countries in the world, males have higher mortality than females at every age. 21,22 The possible reasons for this include the fact that boys are biologically weaker and more susceptible to diseases and premature death, as a result of sex differences in biologic and genetic makeup. 22 This study revealed that across the years under review, more of the deaths occurred at about the month of March, with cumulative deaths highest in the month of March.…”

“…21,22 The possible reasons for this include the fact that boys are biologically weaker and more susceptible to diseases and premature death, as a result of sex differences in biologic and genetic makeup. 22 This study revealed that across the years under review, more of the deaths occurred at about the month of March, with cumulative deaths highest in the month of March. March is usually the start of the rainy season in Uyo, Akwa Ibom State, Nigeria.…”

Comparative Analysis of Variations in Human Sex Ratios of Live Births Among African Populations and at Global Levels

“…Furthermore, analysis of twin sex ratios at birth is not considered in this study, though there could theoretically have different features from the normal single birth delivery in certain societies. For example, in a certain study in Nigeria and Ghana (West Africa), twin birth delivery was believed to have distorted the patterns of the sex ratios at live birth (Pongou 2013). Although the findings of that author could be noted as interesting, the incidence of twin births is too low to affect the overall sex ratios of the live births at country, regional, and worldwide levels.…”