The main result of the paper is that changes
in childhood lead exposure are responsible for a 56% drop in violent crime in the 1990s.
What are those "changes
in childhood lead exposure"? Primarily the move to unleaded gasoline, which happened in the US between 1975 and 1985.
This result is not entirely surprising: I blogged a similar finding, by Rick Nevin, last summer. Nevin’s paper is more international in scope: it covers the
USA, Britain,Canada, France, Australia, Finland, Italy, West Germany, and New Zealand. But it also uses a less rich dataset: the new paper really nails this finding down.
What I learn from this paper is that sometimes the Law of Unintended Consequences can mean unintended positive consequences: the 1970 Clean Air Act had a much more beneficial effect on America than anybody guessed it would at the time. (Today, of course, we’re living in a country where the federal government is suing California not to impose stricter emissions standards on automobiles, which is depressing.)
And as Shankar Vedantam of the Washington Post noted when writing about Nevin last year, these findings make politicians’ claims to have reduced crime much less compelling, especially when you combine them with Steve Levitt’s findings about the effect of abortion on crime. Here’s Wolpaw Reyes:
The elasticity of violent crime with respect to childhood lead exposure is estimated to be
approximately 0.8. This implies that, between 1992 and 2002, the phase-out of lead from gasoline
was responsible for approximately a 56% decline in violent crime… The effect of legalized abortion
reported by Donohue and Levitt  is largely unaffected, so that abortion accounts for a 29%
decline in violent crime (elasticity 0.23), and similar declines in murder and property crime.
Overall, the phase-out of lead and the legalization of abortion appear to have been responsible for
significant reductions in violent crime rates.
Significant? I’ll say. 56% plus 29% is 85%, which means that the overwhelming majority of the reduction in crime can be attributed to exogenous factors for which local politicians can take no credit. Not unless they were involved in the Clean Air Act or Roe vs Wade, anyway.