Science Daily — In a groundbreaking, sure-to-be-controversial new study, Emily Oster (a graduate student in economics at Harvard University) argues that excess female mortality, such as infanticide, may not be the only cause of uncommonly high male to female ratios in many Asian countries. It has long been observed that the relative number of males is higher in certain Asian countries than in the West, where it is close to unity. A number of authors have suggested that this imbalance reflects neglect of female children and poor conditions for women and, as a result, have argued that as many as 100 million women are "missing."

However, in the forthcoming issue of the Journal of Political Economy, Oster proposes an explanation for some of the observed over-representation of males: the hepatitis B virus. She presents new evidence, consistent with existing scientific literature, that carriers of the hepatitis B virus are 1.5 times more likely to have a male child. This evidence includes both cross-country analyses and natural experimentation based on recent vaccination campaigns. In addition, hepatitis B is common in many Asian countries, particularly China, where some 10 to 15 percent of the population is infected.

Using data on viral prevalence by country as well as estimates of the effect of hepatitis on birth sex, Oster concludes that hepatitis B can account for about 45 percent of the "missing women" -- or, more specifically, for as many as 50 percent of the "missing women" in Egypt and West Asia; under 20 percent in Bangladesh, India, Pakistan and Nepal; and around 75 percent of the "missing women" in China.