While police forces around the country have been massively increasing the numbers and size and scope of their weaponry, in the process becoming quasi-military forces without the training that the military receives, violent crime (and teen pregnancies) has been steadily and quite dramatically declining over the last four decades. It would be a mistake to think that this negative correlation has a causal basis in that stronger policing has led to the decline because the decline has been nationwide and benefited areas that changed their policing as well as those that did not.
Kevin Drum wrote last year about the causes for the rise and fall of violent crime and this little nugget of insight about how to identify the nature of the cause of different kinds of epidemics jumped out at me.
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.
This molecule was what General Motors invented in the 1920s to create leaded gasoline that prevented pinging and knocking in high-performance engines.
Drum goes to expand on this idea, using the work of economist and researcher Rick Nevin who in 1994 noticed that the pattern of the rise and decline in violent crime paralleled that of the amount of lead in the atmosphere but lagged it by 23 years. Note that the lead curves and the crime curves in the figure are actually displaced by 23 years, in that the effects of lead show themselves 23 years later.
Nevin postulated that removing lead from gasoline (and paint) has resulted in less lead in the atmosphere and thus less lead getting into the bodies (and brains) of people, especially children, and thus made then less prone to violence when they reached adulthood..
Of course, one swallow does not a summer make and one correlation is hardly persuasive. But Jessica Wolpaw Reyes found a natural experiment within the US that supported this thesis because it turned at that different states in the country implemented reduced lead policies at different rates.
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.
Reyes’s paper can be seen here.
And there was more. Nevin found the same pattern in other countries. As Drum says:
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.”
The pattern persists even at the local level.
Just this year, Tulane University researcher Howard Mielke published a paper with demographer Sammy Zahran on the correlation of lead and crime at the city level. They studied six US cities that had both good crime data and good lead data going back to the ’50s, and they found a good fit in every single one. In fact, Mielke has even studied lead concentrations at the neighborhood level in New Orleans and shared his maps with the local police. “When they overlay them with crime maps,” he told me, “they realize they match up.”
One has to have a healthy skepticism about a simple, single causal factor for the complex set of behaviors that come under the heading of ‘violent crime’. But the case that Drum makes seems quite plausible.