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.

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