WHEN RUDY GIULIANI RAN FOR MAYOR of New York City in 1993, he campaigned on a platform of bringing down crime and making the city safe again. It was a comfortable position for a former federal prosecutor with a tough-guy image, but it was more than mere posturing. Since 1960, rape rates had nearly quadrupled, murder had quintupled, and robbery had grown fourteenfold. New Yorkers felt like they lived in a city under siege.
Throughout the campaign, Giuliani embraced a theory of crime fighting called “broken windows,” popularized a decade earlier by James Q. Wilson and George L. Kelling in an influential article in The Atlantic. “If a window in a building is broken and is left unrepaired,” they observed, “all the rest of the windows will soon be broken.” So too, tolerance of small crimes would create a vicious cycle ending with entire neighborhoods turning into war zones. But if you cracked down on small crimes, bigger crimes would drop as well.Giuliani won the election, and he made good on his crime-fighting promises by selecting Boston police chief Bill Bratton as the NYPD’s new commissioner. Bratton had made his reputation as head of the New York City Transit Police, where he aggressively applied broken-windows policing to turnstile jumpers and vagrants in subway stations. With Giuliani’s eager support, he began applying the same lessons to the entire city, going after panhandlers, drunks, drug pushers, and the city’s hated squeegee men. And more: He decentralized police operations and gave precinct commanders more control, keeping them accountable with a pioneering system called CompStat that tracked crime hot spots in real time.
The results were dramatic. In 1996, the New York Times reported that crime had plunged for the third straight year, the sharpest drop since the end of Prohibition. Since 1993, rape rates had dropped 17 percent, assault 27 percent, robbery 42 percent, and murder an astonishing 49 percent. Giuliani was on his way to becoming America’s Mayor and Bratton was on the cover of Time. It was a remarkable public policy victory.
But even more remarkable is what happened next. Shortly after Bratton’s star turn, political scientist John DiIulio warned that the echo of the baby boom would soon produce a demographic bulge of millions of young males that he famously dubbed “juvenile super-predators.” Other criminologists nodded along. But even though the demographic bulge came right on schedule, crime continued to drop. And drop. And drop. By 2010, violent crime rates in New York City had plunged 75 percent from their peak in the early ’90s.
All in all, it seemed to be a story with a happy ending, a triumph for Wilson and Kelling’s theory and Giuliani and Bratton’s practice. And yet, doubts remained. For one thing, violent crime actually peaked in New York City in 1990, four years before the Giuliani-Bratton era. By the time they took office, it had already dropped 12 percent.
Second, and far more puzzling, it’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.
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.
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?
That tip took Nevin in a different direction. 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.
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: . . .