Hail Emily Oster!

The paper is titled "Hepatitis B Does Not Explain Male-Biased Sex Ratios in China"; here is the abstract:

Earlier work (Oster, 2005) has argued, based on existing medical
literature and analysis of cross country data and vaccination programs,
that parents who are carriers of hepatitis B have a higher offspring
sex ratio (more boys) than non-carrier parents. Further, since a number
of Asian countries, China in particular, have high hepatitis B carrier
rates, Oster (2005) suggested that hepatitis B could explain a large
share (approximately 50%) of Asia’s missing women". Subsequent work
has questioned this conclusion. Most notably, Lin and Luoh (2008) use
data from a large cohort of births in Taiwan and find only a very tiny
effect of maternal hepatitis carrier status on offspring sex ratio.
Although this work is quite conclusive for the case of mothers, it
leaves open the possibility that paternal carrier status is driving
higher sex offspring sex ratios. To test this, we collected data on the
offspring gender for a cohort of 67,000 people in China who are being
observed in a prospective cohort study of liver cancer; approximately
15% of these individuals are hepatitis B carriers. In this sample, we
find no effect of either maternal or paternal hepatitis B carrier
status on offspring sex. Carrier parents are no more likely to have
male children than non-carrier parents. This finding leads us to
conclude that hepatitis B cannot explain skewed sex ratios in China.

We should hold up Emily Oster as a role model of a truth-seeker.  If the abstract does not make it clear, Emily Oster first won her fame by reporting the opposite result about sex ratios.  Here are our previous posts on Emily Oster.

A more general lesson, of course, is simply how difficult it is to get at truth.  This is a well-defined data set with a (more or less) well-defined answer.  Most policy questions aren’t so tractable.