Hepatitis B and sex ratio at birth in Asia

After yesterday's post on sex selection, a reader sent me a link to a BusinessWeek article from earlier this year that discusses a new hypothesis for the elevated proportion of males in many populations:

Many people think the reason [for reduced female birth ratio] 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.

The idea is that hepatitis B infection appears to affect sex ratio at birth among infected vs. noninfected people:

There is much evidence that parents infected by HBV are more likely to have male children. Places with substantial HBV -- Asia, Alaska, and parts of the the former Soviet Union -- tend to have high male-female birth ratios. Studies in Greece and France show that HBV-positive parents had male-female ratios for offsprings of 1.7 to 1.8, vs. 1.1 to 1.2 for those who are HBV-negative. This pattern also shows up among immigrants, with those from high HBV areas, such as China, having high male-female offspring ratios in the U.S.
The biological explanation for the HBV effect is unclear, though it may involve more frequent spontaneous abortion of female fetuses. But the effect is large, concentrated in certain regions, and susceptible to elimination via the HBV vaccine. In Alaska, the use of the HBV vaccine in 1982 led to a sharp decline in high male-female birth ratios.
Among Asian countries, the HBV influence is greatest in China, explaining 75% of Coale's missing women. In India, the adjustment is less important, explaining only 17%. For Asian countries in general, Oster locates 46% of the absent women, ending up with 33 million missing, rather than Coale's 60 million or Sen's 107 million.

The research paper is available online.

This post by Peter Gallagher (no not the While You Were Sleeping actor) raises some skepticism about Oster's conclusions. Although the argument accepts that hepatitis B may explain some of the effect, it gives some reasons to think the influence of hep B is not as strong as Oster predicts.

You may also notice the BusinessWeek column is by a Harvard economist hyping the Ph.D. thesis of new Harvard economics graduate! But there are more interesting reasons for nepotism favoring this particular graduate student, the child of two economists. This Slate article by the Freakonomics guys tells the story with a fascinating connection at the end:

In the early 1980s, a group of psychologists and linguists banded together to write Narratives From the Crib, a study of how children acquire linguistic skills. Narratives was built around the speech patterns of one child, a 2-year-old girl. Her parents had noticed that she often talked to herself in the crib after they said good night and left her room. They were curious to know what she was saying, so they began to record her chatter. They turned on the tape recorder while they were tucking her in and then left it running. Eventually they gave the tapes to a psychologist friend, who shared it with her colleagues. The big surprise to these experts was that the girl's speech was far more sophisticated when she was alone than when she was speaking with her parents. This finding, as Malcolm Gladwell would later write in The Tipping Point, "was critical in changing the views of many child experts."
The 2-year-old girl in question was referred to as Baby Emily. Her full name? Emily Oster.