[The Lone Ranger]

I've heard the claim before, and while the neurotoxic nature of lead is well-established, that the decline in violent crime since the 1970s is largely due to removal of tetra-ethyl lead from gasoline seems to me ... unlikely. It may have some role to play (in fact, I'd be surprised if it didn't, given the well-established behavioral effects of lead exposure), but I'd guess that sociographic factors, demographic trends, and other factors have far more importance.


From what I can tell, the evidence in favor of the claim is suspect at best. Given that there have been no really good epidemiological studies on the subject, and that direct experimental studies would be unethical, there just isn't a robust body of evidence to support the claim.

[It doesn't help that Rick Nevin, one of the more prominent promoters of the claim, got much of his data from Murray and Herrnstein's The Bell Curve -- a ... suspect ... source, to say the least.]



The strongest evidence I've seen in favor of the hypothesis comes from Brazil. My understanding is that leaded gasoline was phased out in São Paulo several years before it was phased out in the rest of Brazil, and that homicide rates in São Paulo have declined dramatically in São Paulo since 2000, while remaining unchanged in the rest of the country. That's intriguing, but hardly an open-and-shut case.



The real problem is that leaded gasoline was around long before the 1970s. In fact, as pointed out in the recent Cosmos series, the average American citizen living in a big city was exposed to quite a lot of lead in the early part of the 20th century. By the 1960s, the toxic nature of lead was becoming widely recognized and there was growing pressure to reduce lead exposure. So, it's likely that someone living in, say, New York City, would have been exposed to much more lead in the 1940s than in the 1960s.

That is, if removal of lead from gasoline in the 1970s sparked a drop in crime, why weren't crime rates so high in the 1920s - 1950s when the average U.S. city-dweller was probably exposed to even higher amounts of lead?