In Business Week Robert Barro writes:
In 1990 my Harvard colleague Amartya Sen caused a stir by observing…that excess female mortality in China, India, and other Asian countries meant that there were 100 million women fewer in the world than there should be. The presumption was that the excess mortality came from discrimination against women by men and governments…[this] shockingly large number became a symbol of discrimination against women in developing countries. Many people think the reason is abortion and the killing of newborn girls. But new research suggests another reason. Harvard economist Emily Oster, in her PhD thesis "Hepatitis B and the Case of the Missing Women" suggests that biology explains a good deal of the missing-women puzzle…
Oster argues that this calculation overlooked something crucial — unusually high male-female birth ratios in Asia years before abortion became widespread…Oster sees the high incidence of the hepatitis B virus (HBV) as a major culprit. There is much evidence that parents infected by HBV are more likely to have male children.
In fact carriers of hepatitis B can have boy-girl ratios as high as 1.55. Oster argues that this factor explains 75% percent of the gender gap in China, albeit only 17% in India. For Asia as a whole, 46% of the gap can be explained.