Freakonomics was an interesting book. It addressed a bunch of unrelated, rather parochial issues, basically on the premise that incentives matter. Steven Landlburg's new book, More Sex, is in that same vein.

The New Republic has an article an the proliferation of Levitt-style analysis, and makes a pretty balanced case for the idea that this can go too far. They note that top Econ journals have since documented that Mexicans pay prostitutes more for unprotected sex (some research there!), diplomats from corrupt countries run up more parking tickets at the UN, people buy too much winter clothing from catalogues when it's really cold, stock traders aren't as sharp on Fridays because they are distracted by the weekend, and the people pay too much for gym memberships they don't use. None of these results are really unexpected, or really important, but all cute. You won't win a John Bates Clarke award for this (Levitt notwithstanding), but have any of those winners from the past 20 years made any models that are really useful in the way biology or chemistry Nobel prize-winning research? I would say not. There's something to be said for Levitt's observation that "I've always been someone who's thought it's better to answer a small question well than to fail to answer a big question."

But cute research is sometimes too cute, and not even true (as I would argue for Levitt's abortion-crime paper).

Perhaps the most infamous example is a paper written by a recent Harvard Ph.D. named Emily Oster. While still an undergraduate, Oster had become fascinated by the so-called "missing women" problem--the hypothesis, attributed to Amartya Sen, that gender discrimination in Asia has created a vast shortage of women. In some cases parents abort daughters, in some cases they commit infanticide, in some cases they simply don't care for their daughters as diligently as they should. Whatever the cause, Sen has suggested there could be as many as 100 million "missing women" in countries like China, India, and Pakistan.

Years later, while wrapping up her Ph.D., Oster stumbled onto a seemingly unrelated fact: a small medical literature suggesting that women with hepatitis B were far more likely to give birth to boys. What followed was a series of sophisticated natural experiments, the upshot of which was to demonstrate that 100 million women hadn't gone missing after all. Instead, unusually high rates of hepatitis B had arranged it so that Asian mothers were producing far more boys than nature's track record would suggest.

It was a fabulously compelling result, one that partially absolved whole societies of lurid crimes against their children. It was also a vindication of the Freakonomics worldview. Levitt published Oster's paper in the Journal of Political Economy. He and his Freakonomics co-author, Stephen Dubner, took to the pages of Slate to breathlessly retell her "economics detective story." And then, just as suddenly, it all fell apart. A snot-nosed grad student from Berkeley pointed out that hepatitis B couldn't possibly explain the missing women problem. It turned out Asian women gave birth to daughters at the same rate as women everywhere else, at least during their first pregnancy. It was only during subsequent births that the ratios changed. Either a bunch of Asian women were running out to get hepatitis B in between their first and second pregnancies, or, as Sen feared, people were taking dramatic steps to avoid ending up with two girls.