Saturday, January 5, 2013

America's Real Criminal Element: Lead

A new piece in Mother Jones magazine by Kevin Drum persuasively argues that the most significant factor in the rise in violent crime rates from the early 1960s to about 1990 was the introduction of lead in gasoline a generation earlier; and the ubiquitous fall in violent crime after 1990 is largely the result of taking lead additives out of gas in the 1970s.

The hero of his story is a researcher named Rick Nevin:

IN 1994, RICK NEVIN WAS A CONSULTANT working for the US Department of Housing and Urban Development on the costs and benefits of removing lead paint from old houses. This has been a topic of intense study because of the growing body of research linking lead exposure in small children with a whole raft of complications later in life, including lower IQ, hyperactivity, behavioral problems, and learning disabilities.
But as Nevin was working on that assignment, his client suggested they might be missing something. A recent study had suggested a link between childhood lead exposure and juvenile delinquency later on. Maybe reducing lead exposure had an effect on violent crime too?

Paint, however, was not the major source of human-induced lead. It was gasoline.

The biggest source of lead in the postwar era, it turns out, wasn't paint. It was leaded gasoline. And if you chart the rise and fall of atmospheric lead caused by the rise and fall of leaded gasoline consumption, you get a pretty simple upside-down U: Lead emissions from tailpipes rose steadily from the early '40s through the early '70s, nearly quadrupling over that period. Then, as unleaded gasoline began to replace leaded gasoline, emissions plummeted.

Intriguingly, violent crime rates followed the same upside-down U pattern. The only thing different was the time period: Crime rates rose dramatically in the '60s through the '80s, and then began dropping steadily starting in the early '90s. The two curves looked eerily identical, but were offset by about 20 years.

Nevin found that 90 percent of the variation in violent crime is explained by lead emissions from motor vehicles:

So Nevin dove in further, digging up detailed data on lead emissions and crime rates to see if the similarity of the curves was as good as it seemed. It turned out to be even better: 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.

If lead exposure were as important a factor to explain the variation in violent crime rates as Nevin believed, the changes he found in the United States would have to be found in other countries, as well. They were:

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."

A study not discussed in Mr. Drum's article--perhaps it does not exist--is whether, if you compare a control group of violent criminals with a similar group of people (same age, gender, region, etc.), you would find that the violent criminals have substantially more bodily evidence of lead exposure. My bet is you would find just that.

TO CLARIFY ... Mr. Nevin's contention is not that lead exposure is the only cause or even the most important cause of violent crime. It's that widespread lead exposure is the most important cause of the rise in violent crime over the period violent crime rose from a base level to its peak rates; and that the great reduction of lead exposure beginning in the mid-1970s is the most important cause of the decline in violent crime rates from 1990 to the present.

That suggests we will hit a new base level, if we have not hit that already, and thus, unless something else changes, violent crime rates will stop falling. Also, if Nevin's lead theory is right, there is no reason to think we will again see violence rates like those seen in the 1970s and '80s.

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