Several of you have e-mailed a link to Kevin Drum’s MoJo piece about the possible connection between environmental lead levels and the drop in criminality. I just read it, and it really is as provocative and as interesting as people say. Drum begins with a survey of various theories different experts have forwarded to explain the dramatic decrease in crime over the past two decades. Excerpt:

But there’s a problem common to all of these theories: It’s hard to tease out actual proof. Maybe the end of the crack epidemic contributed to a decline in inner-city crime, but then again, maybe it was really the effect of increased incarceration, more cops on the beat, broken-windows policing, and a rise in abortion rates 20 years earlier. After all, they all happened at the same time.

To address this problem, the field of econometrics gives researchers an enormous toolbox of sophisticated statistical techniques. But, notes statistician and conservative commentator Jim Manzi in his recent book Uncontrolled, econometrics consistently fails to explain most of the variation in crime rates. After reviewing 122 known field tests, Manzi found that only 20 percent demonstrated positive results for specific crime-fighting strategies, and none of those positive results were replicated in follow-up studies.

So we’re back to square one. More prisons might help control crime, more cops might help, and better policing might help. But the evidence is thin for any of these as the main cause. What are we missing?

Experts often suggest that crime resembles an epidemic. But what kind? Karl Smith, a professor of public economics and government at the University of North Carolina-Chapel Hill, has a good rule of thumb for categorizing epidemics: If it spreads along lines of communication, he says, the cause is information. Think Bieber Fever. If it travels along major transportation routes, the cause is microbial. Think influenza. If it spreads out like a fan, the cause is an insect. Think malaria. But if it’s everywhere, all at once—as both the rise of crime in the ’60s and ’70s and the fall of crime in the ’90s seemed to be—the cause is a molecule.

A molecule? That sounds crazy. What molecule could be responsible for a steep and sudden decline in violent crime?

Well, here’s one possibility: Pb(CH2CH3)4.

That is, lead. The gist of Drum’s piece is that research indicates that the crime drop occurred after leaded gasoline began to be phased out. It didn’t happen overnight, but if you account for a lag time, the numbers add up:

In a 2000 paper (PDF) he concluded that if you add a lag time of 23 years, lead emissions from automobiles explain 90 percent of the variation in violent crime in America. Toddlers who ingested high levels of lead in the ’40s and ’50s really were more likely to become violent criminals in the ’60s, ’70s, and ’80s.

And with that we have our molecule: tetraethyl lead, the gasoline additive invented by General Motors in the 1920s to prevent knocking and pinging in high-performance engines. As auto sales boomed after World War II, and drivers in powerful new cars increasingly asked service station attendants to “fill ‘er up with ethyl,” they were unwittingly creating a crime wave two decades later.

The great thing about the way Drum writes about this is that he keeps trying to come up with alternate explanations — the same things that probably occur to you, as a skeptical reader, as you go along. I found myself thinking, “What about in other countries? Have they found a correlation between environmental lead from gasoline, and crime rates there? I bet they haven’t.” Then I read this in Drum’s piece:

Meanwhile, Nevin had kept busy as well, and in 2007 he published a new paper looking at crime trends around the world (PDF). This way, he could make sure the close match he’d found between the lead curve and the crime curve wasn’t just a coincidence. Sure, maybe the real culprit in the United States was something else happening at the exact same time, but what are the odds of that same something happening at several different times in several different countries?

Nevin collected lead data and crime data for Australia and found a close match. Ditto for Canada. And Great Britain and Finland and France and Italy and New Zealand and West Germany. Every time, the two curves fit each other astonishingly well. When I spoke to Nevin about this, I asked him if he had ever found a country that didn’t fit the theory. “No,” he replied. “Not one.”

Really, do go and read the whole thing.  Drum also presents evidence that lead remains in the environment in the poorest zip codes in American cities, as well as explains what scientists already know about how lead affects the brain. Excerpt:

So lead is a double whammy: It impairs specific parts of the brain responsible for executive functions and it impairs the communication channels between these parts of the brain. For children like the ones in the Cincinnati study, who were mostly inner-city kids with plenty of strikes against them already, lead exposure was, in Cecil’s words, an “additional kick in the gut.” And one more thing: Although both sexes are affected by lead, the neurological impact turns out to be greater among boys than girls.

Other recent studies link even minuscule blood lead levels with attention deficit/hyperactivity disorder. Even at concentrations well below those usually considered safe—levels still common today—lead increases the odds of kids developing ADHD.

In other words, as Reyes summarized the evidence in her paper, even moderately high levels of lead exposure are associated with aggressivity, impulsivity, ADHD, and lower IQ. And right there, you’ve practically defined the profile of a violent young offender.

Drum is careful not to draw definitive conclusions, but he provides more than enough reason to take the lead hypothesis seriously. And he makes a valuable point that many of us will be reluctant to do so because the lead hypothesis may mean that our pet theories explaining the crime rate won’t stand up if lead is the main culprit. He brings up early in the piece the sociologist John DiIulio’s prediction in the 1990s that a new generation of antisocial “super predators” were being raised in the inner cities, and would plague us all. DiIulio is a serious guy, and I well remember being frightened by his claim — precisely because it made sense to me, given what we know about the connection between fatherlessness and juvenile delinquency.

But it didn’t happen. Why didn’t it happen? The lead hypothesis, if true, would go a long way toward explaining that. This is not to say that family formation and culture have nothing to do with criminality, any more than a favored liberal explanation for inner-city crime — the lack of jobs — has nothing to do with criminality. It is to suggest, though, that they have less to do with it than people like me think.

UPDATE: Steve Sailer voiced skepticism about this theory five years ago (thanks, readers).