Why Is Infant Mortality Higher in Boys Than in Girls? A New Hypothesis Based on Preconception Environment and Evidence F
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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 Roland Pongou # Population Association of America 2012
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. Keywords Preconception origins hypothesis . Sex differences in mortality . Preconception environment . Child biology . Twins
Introduction The long-observed greater chance of survival for female children than for male children (Graunt 1662) has been explained by sex differences in genetic and biological makeup, with male children being biologically weaker and more susceptible to diseases than their female counterparts (e.g., Naeye et al. 1971; Waldron 1983). A
R. Pongou (*) Department of Economics, University of Ottawa, Ottawa, ON K1N 6N5, Canada e-mail: [email protected]
R. Pongou
common assumption in studies testing this biological hypothesis is that the offspring sex ratio1 is randomly assigned across and within families. Recent studies, however, have revealed that it is not random. For example, girls are more likely to be born to smoking parents (Fukuda et al. 2002); to mothers with low gestational weight (Cagnacci et al. 2004); to aged parents (Jacobsen et al. 1999; Juntunen et al. 1997); and to parents with certain medical conditions, such as type 1 diabetes (James 1998a; Rjasanowski et al. 1998). An offspring sex ratio biased toward males has been observed among wealthy parents (Almond and Edlund 2007; James 1998b; Trivers and Willard 1973), men with multiple sclerosis, and men exposed to certain environmental toxins (James 1998b). As di
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