The fact that millions of females are “missing” in East Asia and South Asia has been attributed to cultural factors that support strong son preference in these countries. A widely disseminated paper by Emily Oster argues that a large part of this phenomenon can be attributed to excessively masculine sex ratios at birth resulting from maternal infection with hepatitis B. If her thesis is true, current policies to address this problem would need to be reframed to include biological factors in addition to cultural factors. The data show, however, that whether or not females “go missing” is determined by the existing sex composition of the family into which they are conceived. Girls with no older sisters have similar chances of survival as boys. However, girls conceived in families that already have a daughter experience steeply higher probabilities of being aborted or of dying in early childhood. This indicates that cultural factors still provide the overwhelming explanation for the “missing” females.