Should we thank unleaded gas and the EPA for the great modern crime decline?

The provocative question in the title of this post comes from this new provocative Mother Jones article by Kevin Drum headlined "America's Real Criminal Element: Lead." The lengthy piece is probably the first "must read" of 2013 for crime and punishment fans, and here are a few excerpts which highlight why:

[I]t's not just New York that has seen a big drop in crime. In city after city, violent crime peaked in the early '90s and then began a steady and spectacular decline. Washington, DC, didn't have either Giuliani or Bratton, but its violent crime rate has dropped 58 percent since its peak. Dallas' has fallen 70 percent. Newark: 74 percent. Los Angeles: 78 percent. There must be more going on here than just a change in policing tactics in one city. But what?

There are, it turns out, plenty of theories.  When I started research for this story, I worked my way through a pair of thick criminology tomes. One chapter regaled me with the "exciting possibility" that it's mostly a matter of economics: Crime goes down when the economy is booming and goes up when it's in a slump. Unfortunately, the theory doesn't seem to hold water — for example, crime rates have continued to drop recently despite our prolonged downturn.

Another chapter suggested that crime drops in big cities were mostly a reflection of the crack epidemic of the '80s finally burning itself out. A trio of authors identified three major "drug eras" in New York City, the first dominated by heroin, which produced limited violence, and the second by crack, which generated spectacular levels of it. In the early '90s, these researchers proposed, the children of CrackGen switched to marijuana, choosing a less violent and more law-abiding lifestyle. As they did, crime rates in New York and other cities went down.

Another chapter told a story of demographics: As the number of young men increases, so does crime. Unfortunately for this theory, the number of young men increased during the '90s, but crime dropped anyway. There were chapters in my tomes on the effect of prison expansion. On guns and gun control. On family. On race. On parole and probation. On the raw number of police officers. It seemed as if everyone had a pet theory. In 1999, economist Steven Levitt, later famous as the coauthor of Freakonomics, teamed up with John Donohue to suggest that crime dropped because of Roe v. Wade; legalized abortion, they argued, led to fewer unwanted babies, which meant fewer maladjusted and violent young men two decades later.

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....

Even low levels have a significant effect. 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?...

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....

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....

During the '70s and '80s, the introduction of the catalytic converter, combined with increasingly stringent Environmental Protection Agency rules, steadily reduced the amount of leaded gasoline used in America, but [researcher Jessica Wolpaw] Reyes discovered that this reduction wasn't uniform.  In fact, use of leaded gasoline varied widely among states, and this gave Reyes the opening she needed.  If childhood lead exposure really did produce criminal behavior in adults, you'd expect that in states where consumption of leaded gasoline declined slowly, crime would decline slowly too.  Conversely, in states where it declined quickly, crime would decline quickly.  And that's exactly what she found.

Meanwhile, [researcher Rick] Nevin ... in 2007 he published a new paper looking at crime trends around the world. 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."

We now have studies at the international level, the national level, the state level, the city level, and even the individual level. Groups of children have been followed from the womb to adulthood, and higher childhood blood lead levels are consistently associated with higher adult arrest rates for violent crimes. All of these studies tell the same story: Gasoline lead is responsible for a good share of the rise and fall of violent crime over the past half century.

Like many good theories, the gasoline lead hypothesis helps explain some things we might not have realized even needed explaining. For example, murder rates have always been higher in big cities than in towns and small cities. We're so used to this that it seems unsurprising, but Nevin points out that it might actually have a surprising explanation—because big cities have lots of cars in a small area, they also had high densities of atmospheric lead during the postwar era. But as lead levels in gasoline decreased, the differences between big and small cities largely went away. And guess what? The difference in murder rates went away too. Today, homicide rates are similar in cities of all sizes. It may be that violent crime isn't an inevitable consequence of being a big city after all.

The gasoline lead story has another virtue too: It's the only hypothesis that persuasively explains both the rise of crime in the '60s and '70s and its fall beginning in the '90s. Two other theories — the baby boom demographic bulge and the drug explosion of the '60s — at least have the potential to explain both, but neither one fully fits the known data. Only gasoline lead, with its dramatic rise and fall following World War II, can explain the equally dramatic rise and fall in violent crime.

Some related posts on the great modern crime decline: 

• Still more (and still puzzling) crime rate declines reported by FBI
• Effective Washington Post commentary talks up great (and still puzzling) crime decline
• Amazingly great new FBI data: crime down yet again in start of 2011!
• Still more great news and data on the latest crime rates in the United States
• Remarkable drop in US violent crimes rates in 2010 according to latest BJS data
• Wonderfully puzzling violent crime rate continue to decline (despite NFL lockout)
• Some speculations about the great crime decline in Florida
• Despite death penalty's practical demise and a prisoner release order, California crime hit record low in 2010
• Is the great US crime decline now finally over?: BJS reports crime up in 2011
• FBI reports crime was down yet again in 2011 (though BJS said it was up)