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.


Role model is right. I once heard Emily say that we (meaning economists) should be about trying to find the truth, and I now believe her.

Its nice to think she was just investigating this topic with no direct purpose, but I think the 1 child per family law is the reason. China probably said there are so many more men than women because of hepatitis B, because thats how the government tried to control the situation, by placing blame on something it cannot directly control. The truth is, the Chinese have been murdering their female children in an effort to get the much more valuable Male offspring.

As a physicist I approve wholeheartedly of the capability to contradict your earlier work with new evidence. In fact, I would go beyond saying that you should not be wedded to your beliefs about the validity of certain theories or models; rather, I think you should be theory-agnostic. I do work in particle theory, but I have no real faith at all in the existence (or non-existence) of the Higgs boson. Even in a discipline such as mine where ideology is not really a concern, you see far too many scientists with pet theories for which there is no real evidence...

I agree that we should not be wedded to our models or theories in the face of contradictory empirical evidence. However, we cannot be theory-agnostic when using the tools of micro-econometrics. Virtually all the techniques assume that there is a theory and it has given you a prediction about a ceteris paribus relationship and that is what you are testing. When you run the empirical test you are relying on your underlying model to help you pick the proper specification. In most empirical quantitative social science that comes from the econ world our tools are built to test predictions from models.

PLW - witches was something she published from college, but she was a "star" because of HepB and the missing women.

Matt - I agree wholeheartedly. I think you see the same thing in economics, actually. Ideology matters much less in 2008 than it probably did in 1948, but pet theory biases abound. When I teach a research methods class, I plan to take my class through this sex ratio material that Emily's work fits into. I think students need to learn this as much as identification as soon as possible.

Okay, but how about giving some credit to all the non-economists who thought her initial hypothesis was silly? The evolutionary scientists I stay in touch with dismissed her theory quickly.

Steve, there are plenty of people who "specialize in the economy." They're called macroeconomists, and it's always the hottest area. But there has been for decades now also economists who think all of human behavior, and not just "economic" behavior, can be fruitfully studied with the tools that were partly borne out of economic theory. Visit political science and sociology departments, and you'll increasingly see game theory and rational choice crowding out the older methods. Or at least, having a place therein.

Here's a current example of economists' negligence of their basic responsibilities.

The freakonomists claim that they're good at catching cheating, such as in sumo tournaments. Yet, massive mortgage fraud went on across America in 2005 and 2006 involving "liar loans," which now threaten to plunge America into hard times. How good a job did economists, with their cheater detection tools, do of noticing and publicizing and heading off this historic catastrophe?

There are two somewhat different questions here:

First, if you want to get a serious understanding of another field, like Paul Krugman wanted to get to understand evolutionary biology in the 1990s, you have to do some heavy reading over a lengthy period of time. Krugman worked hard and found out that most of the stuff he thought he knew about Darwinism from reading Stephen Jay Gould was misleading, and that Gould was the John Kenneth Galbraith of biology.

Second, if you just want to see if your brainstorm idea is worth investing your time and other people's time in it, then do reality checks.

There are a lot of ways to do reality checks, such as asking experts.

Several years ago, I asked Greg Cochran, who knows far more about infectious diseases than any economist, about Emily Oster's theory and he didn't think her idea was worth pursuing.

Similarly, when a certain famous young Harvard economist was profiled by Stephen Dubner in the NYT Magazine as the next genius, Dubner devoted much space to the economist's theory that the reason African-Americans suffer so much from hypertension was that a slave with "a higher capacity for salt retention might also retain more water and thus increase his chance of surviving" the Middle Passage in slave ships.

So, I put this economist in touch via email with Cochran, who explained to him the arithmetic that shows that even the strongest natural selection acting on a single generation of ancestors can't account for the big genetic differences in salt retention between African-Americans and white Americans. Instead, it was the difference between the climate in Africa and in Europe, working over hundreds of generations, that selected for different gene variants.

But this economist didn't want to hear about it and cut off the dialogue.

The main thing is -- don't just talk to other economists. For example, Steven Levitt took his abortion-cut-crime theory comparing crime rates in 1985 and 1997 around to seminars in leading departments of economics in 1998-99. And, apparently nobody pointed out to Levitt that, in contradiction to his theory, in between 1985 and 1997, serious crime among young people (born right after Roe v. Wade) had soared, the opposite of what his theory predicted. All these economists had forgotten (or never noticed) the Crack Years.

Economists really aren't that good at remembering facts -- that's why they like theories. Conversely, historians are good at remembering facts, but they aren't very good at theories. Everybody has different strengths and weaknesses.

The bottom line is you have to get out of your comfort zone and go mix it up with people who don't think like you do.

Glad to hear that you are so opposed to people talking about areas in which they have so specialty or training in. I hope that means you will stop criticizing econometric techniques and empirical specification choices that you have no knowledge whatsoever about.

Of course you have pet theories about race, marriage, crime and genetics all things which you have absolutely no training or expertise in. Every time you write an op-ed or blog post exposing your made-up explanation about whatever the topic of the day is you are committing the same sin that you always accuse economist of. The thing is that at least these economist have the skills to rigorously test the theories they come up with, you don't even have that.

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