Friday, September 30, 2005

Ceteris paribus 

James Taranto thinks the reaction to Bill Bennett's comments on the relation of abortion and crime indicates maybe the end of political correctness. He does so by actually reading Steven Levitt, whose work was the impetus for Bennett's remarks. But Levitt points out that Bennett (and by extension, Taranto) have misread him.
It is true that, on average, crime involvement in the U.S. is higher among blacks than whites. Importantly, however, once you control for income, the likelihood of growing up in a female-headed household, having a teenage mother, and how urban the environment is, the importance of race disappears for all crimes except homicide. (The homicide gap is partly explained by crack markets). In other words, for most crimes a white person and a black person who grow up next door to each other with similar incomes and the same family structure would be predicted to have the same crime involvement. Empirically, what matters is the fact that abortions are disproportionately used on unwanted pregnancies, and disproportionately by teenage women and single women.
We are forever explaining to students of economics the value of the term ceteris paribus or "all other things being equal." Since we don't have test tubes or other treatments, we have to do it theoretically through a mental exercise or empirically using regression analysis. This is the kind of thing economists argue about frequently -- did you include all the right variables to be able to invoke ceteris paribus and claim the relationship? This type of problem, called specification bias, is explained by Peter Kennedy (who wrote one of the best books for teaching econometrics to undergraduates). See also these notes.

Levitt and Dubner, in Freakonomics, note that former Minneapolis police chief and gubernatorial candidate Anthony Bouza, who wrote a book including the argument that abortion was
"arguably the only effective crime-prevention device adopted in this nation since the late 1960s." When Bouza's opinion was publicized just before the election, he fell sharply in the polls. And then he lost.
If somehow NARN replaces Bennett in morning drive, nothing will have changed.