Want to Cut Crime? It Takes a Neighborhood.

By Tim Harford
Sunday, February 24, 2008

On a sticky Washington evening two summers ago, my wife and I were strolling down 15th Street NW, near Whole Foods. We were licking ice cream cones and chatting as our 13-month-old daughter wriggled in a carrier on my back. Then a scream from across the street snapped us out of our conversation. Looking up, I saw a man chasing a woman around a car that had come to a sudden halt in the middle of the street.

For a moment I thought it was all in fun -- we were eating ice cream, after all -- until the woman fell to the ground and the man punched her as hard as he could. A quicker-thinking bystander sprinted through the screeching traffic and barreled into the assailant, momentarily knocking him away from the prone woman. My wife and I looked at each other. All she said was "Tim," but what she meant was, "What are you going to do?"

I handed her the baby and trotted across the street, the baby carrier flapping on my back. Up close, I realized that I might be making a big mistake: All three people in the struggle were slick with blood. There was a knife somewhere. Worse, I had no idea where it was.

My main contribution to the melee was an undignified yank on the knifeman's clothes, which may have helped pull him away from the stricken woman but mostly succeeded in straining my lower back. No matter: Within a few seconds, half a dozen men were sitting on the attacker. I found myself crouching by the young victim's side, my gaze alternating between her eyes and the awful knife wounds on her arm and in her belly. I could hardly believe that she might survive.

She did. The neighborhood itself had saved her.

These days, I live in London, close to a park where, not long ago, a young American woman out for her morning run also suffered a knife attack. The outcome was very different: Nobody helped, she died, and the murderer has not been caught.

Two wholly random assaults -- the D.C. attacker later told police that he didn't know his victim and had been taking drugs to celebrate his 21st birthday -- with equally random outcomes. Life and death seemed arbitrary.

But over the months that followed, I found the 15th Street experience infiltrating my life as a writer and an economist. I was working on a book about how economics can help us find the hidden logic in aspects of life that seem complex and unpredictable. So could economics say anything about what makes our neighborhoods defend us or abandon us?

Odd as it may sound, I discovered that it could. Economists can now tell us why neighborhoods go through dramatic transitions from dangerous to safe or rich to poor; they have established a clear link between urban architecture and crime; they can even shed some light on whether local crime is contagious. And they can tell us what difference law enforcement really makes when the streets are peopled by those who try to kill for no reason.

Nobody who has lived in the District needs to be told that cities frequently fall into a sharply defined patchwork of thriving areas and struggling ones, often divided along racial lines. It is easy to feel that this oil-and-vinegar separation is the result of bitter prejudice. But the Nobel Prize-winning economist Thomas C. Schelling proved decades ago that while the segregation may be grim, the motivations that lead to it may be less entrenched than you might suppose.

In the days before complex computer simulations, Schelling demonstrated his theory with a little game played with randomly distributed pennies and nickels on a checkerboard. He invented a simple rule for how the coins moved: A nickel might be happy touching two or more other nickels, but if it touched just one other nickel, it would hop elsewhere, leaving its former neighbor isolated. One coin after another would move in a chain reaction. Schelling's game seemed to make possible a mixed checkerboard, but the result was always segregation.

The lesson? Even if everyone were comfortable living in a mixed neighborhood, extreme segregation -- by race, class or income -- could still emerge from people's mild preferences not to be outnumbered. Countless individually rational decisions can snowball into a socially regrettable outcome.

Now armed with computer simulations, Schelling's successors are using his insight to explore ways in which societies can "flip" from bad situations to good ones. City streets can be unsettlingly empty or reassuringly thronged with passersby. The safer and livelier the streets feel, the safer and livelier they become. This virtuous circle means that a small catalyst can transform a neighborhood from struggling to thriving. Thanks to Whole Foods and a few bars on 14th and 17th streets, the 15th Street neighborhood went through exactly one of those transformations not long ago -- and the young woman I saw being attacked owes her life to it.

Urban architecture matters, too. This is something we feel intuitively but find hard to prove or to quantify. Think of high-rise apartments. Do they make a city safer by packing more people into an area and giving the streets a greater bustle? Or are cities safer if most buildings are low-rise, so that people feel a connection to the street?

Two new-wave economists, Edward Glaeser of Harvard and Bruce Sacerdote of Dartmouth, matched crime figures with data on building height and discovered that the residents of high-rise apartments are much more likely to be victims of crime -- specifically street crime. The effect remains similar after statistically adjusting for poverty, demographics and public housing: It's the height of the building itself that matters.

This is evidence in favor of urbanist Jane Jacobs's persuasive -- but unproven -- insight that "eyes on the street" are what make a neighborhood safe. The hero who sprinted across the street to knock the knife away was sitting on the stoop outside his home, not gazing out his penthouse window.

We also know much more than we did 10 years ago about the role of law enforcement in keeping our cities safe. Steven Levitt of the University of Chicago -- now famous as the co-author of "Freakonomics" -- used state-by-state differences in the juvenile punishment system to show that teenage delinquents paid close attention to the threat of punishment. The day they passed the age of majority was the day they cleaned up their act -- an effect that was more pronounced in states where the adult penal system was dramatically harsher than the juvenile one. The economists Alex Tabarrok and Jonathan Klick recently found that crime dropped noticeably in the District when extra police were placed on duty because of terrorism alerts. In both cases, the result is not a surprise -- you'd expect tough sentences and more cops to deter crime -- but the researchers were able to tell policymakers how big the effect was.

Economists have even tried to figure out how much your neighborhood affects your chances in life, mining data from the federal government's "Moving to Opportunity for Fair Housing" scheme, which -- starting in the mid-1990s -- offered randomly chosen residents of public housing the chance to move to a wealthier neighborhood. Economists such as Jeffrey R. Kling of the Brookings Institution determined that a fresh start makes people healthier, happier and less likely to become victims of crime. But it is not a panacea; those who move are no more likely to get jobs or stay out of trouble with the police.

Some of these discoveries are things we suspected but didn't know or forces we could identify but not quantify. Taken as a whole, they're still a mere handful of jigsaw pieces from a very large puzzle. But they are credible explanations discovered in the face of the apparent randomness of everyday life. While the economics profession won't rescue the inner cities by itself, policymakers should take note, making decisions about policing, public housing and planning approvals in light of the evidence.

Our cities remain vital centers. New ideas are concentrated there; their environmental footprint is low because more residents live in small apartments and travel on foot; and young people still flock there in search of freedom and excitement. We need to make these great cooperative machines work better. Our city neighborhoods have a logic to them, and it's a logic we should try to understand. Our neighborhoods -- sometimes even our lives -- depend on it.

Tim Harford, a Financial Times columnist, is the author of "The Logic of Life: The Rational Economics of an Irrational World."


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