Yet as far as Theories of Everything go, lead-caused-the-60s-crime-wave is one of the less derisible. Lead is bad for you, and it might have had noticeable effects on society as a whole.

The problem is coming up with ways to test the theory. A half-dozen years ago, I blogged (”Lead Poisoning and the Great American Freakout”) about the research that Drum finds so convincing today. One reality check immediately suggested itself: Back in the late 1960s, densely populated Japan was notorious for automobile-induced air pollution. Yet crime didn’t rise in Japan. The country remained an orderly, intelligent, non-impulsive culture.

That’s one strike against the theory. Another problem is that Jessica Wolpaw Reyes’s attempt to correlate small differences from when American states began phasing out leaded gasoline in the 1970s to when crime began declining in the 1990s isn’t convincing to many besides Drum. Reyes came up with statistically significant results for total violent crimes, but not for homicides (the most accurately counted crime), nor for property offenses.

Yet two strikes isn’t bad for a causes-of-crime theory. It holds up better than the famous Freakonomics abortion-cut-crime surmise.

Exactly how deleterious this metal is at low levels, at what ages, and through which modes of transmission remains murky. This doesn’t mean that lead wouldn’t cause increased crime by lowering IQ, all else being equal. But perhaps lead poisoning also diminishes crime by making its victims more sluggish, lacking in the initiative to go out and commit felonies.

On the other hand, perhaps lead pollution can help explain a little of the differences we see in black homicide rates between cities.

After Hurricane Katrina in 2005, I was widely denounced for pointing out that the bad behavior visible on television wasn’t necessarily representative of average African Americans. You shouldn’t assume the typical black would behave as badly as New Orleans’s blacks, who had been more homicidal than most other cities’ blacks for years, even before Katrina.

This was especially true of the Crescent City’s flood-prone and poverty-stricken Lower Ninth Ward neighborhood. Back in 2005, I speculated that the traditional high crime rates of New Orleans’s blacks were due to the city’s let-the-good-times-roll culture being unsuited for African Americans, who need more socially conservative cultures.

Yet perhaps lead plays a role in New Orleans as well. Heavy metals would tend to build up in the Lower Ninth Ward’s below-sea-level soil.

Notoriously, scientific papers end with a pronouncement that More Research Is Needed. It’s been six years since I first pointed out some of the flaws in the papers upon which Drum relies. Yet for all their weaknesses, they remain the state of the art in thinking about the impact of lead pollution on crime, a subject with interesting implications for real-estate investors.

If the ongoing decline in lead in the environment will cause crime rates to drop in the future, then more slums become plausibly gentrifiable. Consider the vast expanse of South-Central Los Angeles, with its superb climate and ample parking. If black and Hispanic crime rates drop in the future, South-Central could begin to look attractive to adventurous white and Asian gentrifiers.

But if a stronger link is someday established between lead pollution and criminal activity among children, that might make some currently gentrifying neighborhoods less attractive, with unfortunate implications for investors.

So if there are connections between lead poisoning and crime, there’s gold to be made in unraveling them.