A recent study challenges the assumption that the large deficit of girls in East and South Asia reflects the preference for sons, suggesting that much of the deficit—as much as 75 percent in China—is attributable to hepatitis B (HBV). The claim is inconsistent with the results of a study based on a large medical data set from Taiwan (China), which indicates that HBV infection raises a woman's probability of having a son by only 0.25 percent. In addition, demographic data from China show that the only group of women who have elevated probabilities of bearing sons are those who have already borne daughters. This pattern makes it difficult to see how any biological factor can explain a large part of the imbalance in China's sex ratios at birth, unless it can be shown that it somehow selectively affects those who have borne girls or causes them to first bear girls and then boys. The Taiwanese example suggests that this is not the case with HBV, the impact of which is unaffected by the sex composition of previous births. The data thus support the cultural rather than the biological explanation for gender imbalance.