Do Demographics Explain the Latest Drop in Crime Rates?

The Federal Bureau of Investigation has just issued its latest crime report and the New York Times reports that the data reveal a drop in crime in the US:

The number of violent crimes in the United States dropped significantly last year, to what appeared to be the lowest rate in nearly 40 years, a development that was considered puzzling partly because it ran counter to the prevailing expectation that crime would increase during a recession.

All of the experts interviewed for the story were baffled by this trend, and that immediately called to my mind the highly controversial theory put forth by Steven Levitt and Stephen Dubner in their very popular book Freakonomics. In Chapter 4 of that book they argue that:

In the early 1990s, just as the first cohort of children born after Roe v. Wade was hitting its late teen years—the years during which young men enter their criminal prime—the rate of crime began to fall. What this cohort was missing, of course, were the children who stood the greatest chance of becoming criminals. And the crime rate continued to fall as an entire generation came of age minus the children whose mothers had not wanted to bring a child into the world. Legalized abortion led to less unwantedness; unwantedness leads to high crime; legalized abortion, therefore, led to less crime.

The interesting thing about that theory is, of course, that it fits with the demographic idea that changing cohort size and characteristics make a difference in society. If we look back approximately twenty years ago from now, what demographic factors can we find that might help us explain a drop in crime? One possibility is the steep decline in teenage birth rates among all groups in American society. Birth rates among 15-17 year old girls were 45% lower in 2008 than in 1991 and 26% lower among 18-19 year olds. Since we can assume that a large fraction of babies to girls that age are unwanted, a drop in the birth rate to young mothers will, paraphrasing Levitt and Dubner's words, lead to less unwantedness; unwantedness leads to high crime; lower teen birth rates, therefore, lead to less crime. 

This conclusion will have to wait for more detailed investigation, but in the meantime, some NFL football players are concerned that the crime rate will go up if they aren't allowed to play this fall...