The theory offered by the economist, Rick Nevin, is that lead poisoning accounts for much of the variation in violent crime in the United States. It offers a unifying new neurochemical theory for fluctuations in the crime rate, and it is based on studies linking children's exposure to lead with violent behavior later in their lives.
What makes Nevin's work persuasive is that he has shown an identical, decades-long association between lead poisoning and crime rates in nine countries.
"It is stunning how strong the association is," Nevin said in an interview. "Sixty-five to ninety percent or more of the substantial variation in violent crime in all these countries was explained by lead." ...
"It is startling how much mileage has been given to the theory that abortion in the early 1970s was responsible for the decline in crime" in the 1990s, Nevin said. "But they legalized abortion in Britain, and the violent crime in Britain soared in the 1990s. The difference is our gasoline lead levels peaked in the early '70s and started falling in the late '70s, and fell very sharply through the early 1980s and was virtually eliminated by 1986 or '87.
It would be interesting to see Nevin's new data. Here's an old article. (I haven't read it yet.) And here's a newspaper article about another researcher's lead-IQ-crime connection.
Abortions can be assigned to very precise times, which quickly allowed big doubts to be raised about Steven D. Levitt's abortion-cut-crime theory, since the cohort born soon after legalization had much higher murder rates. Lead is a little slipperier to analyze, because it hangs around in the environment, perhaps providing wiggle room for the analyst if the chronology doesn't quite match up.
The WaPo article ignores Nevin's link of lead to lowering IQ, as in this 2000 article. That ingesting lead makes you stupider was known for a long time. In James T. Farrell's 1930s novels about Chicago prole Studs Lonigan, Studs and his pals debate whether to give up the good pay of working as painters because all the old painters seem pretty dim from exposure to lead paint.
Meanwhile, Modern Dragons offers another potential source of influence on human behavior, one that's been analyzed even less: gut flora.