Want to Cut Crime? It Takes a Neighborhood.

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."

undercovereconomist@gmail.com