The first time that David Kirk visited New Orleans after Hurricane Katrina was at the end of 2005. His in-laws were from the city. Kirk and his wife visited them at Christmas, just four months after the storm hit, and then went back again on several more occasions throughout 2006. New Orleans was devastated. Thousands had fled. “I’ll admit I’d drive around the Lower Ninth, taking it all in, feeling a little guilty about being the gawking tourist,” Kirk said not long ago. “It made an impression on me. These neighborhoods were gone.”

Kirk is a sociologist at the University of Oxford. He trained at the University of Chicago under Robert Sampson, and, for Sampson and the small army of his former graduate students who now populate sociology departments around the world, neighborhoods are the great obsession: What effect does where you live have on how you turn out? It’s a difficult question to answer because the characteristics of place and the characteristics of the people who happen to live in that place are hard to untangle. As Kirk drove around the Lower Ninth, however, he realized that post-Katrina New Orleans provided one of those rare occasions when fate had neatly separated the two variables. In the course of bringing immeasurable suffering to the people of New Orleans, Katrina created what social scientists call a “natural experiment”: one day, people were in the neighborhoods where they had lived, sometimes for generations. The next day, they were gone—sometimes hundreds of miles away. “They had to move,” Kirk said. What, he wondered, were the implications of that?

“I worked my connections to see who would talk to me,” Kirk went on. “It turned out that one of my colleagues at the University of Maryland had done research on boot camps in Louisiana. Ultimately, I got in touch with someone who is now the head of the prison system, a guy named James LeBlanc.” Kirk’s idea was to look at convicted criminals from New Orleans who had been released from prison after Katrina. As a group, they were fairly homogeneous: largely black, largely poor. For years, their pattern was to return to their old neighborhoods after they were released: to their families, homes, social networks. But for some, by the most random of circumstances, that was now impossible. Their neighborhoods—the Lower Ninth, New Orleans East—had been washed away. How did the movers compare with the stayers?

“This was December, 2006,” Kirk recounted. “I asked for a few things. I wanted information on where prisoners were living prior to Katrina. And I also wanted information on where they were living after release. I basically got an address file from the Department of Corrections for everyone who came out of prison from 2001 to 2007. It was extremely messy. They don’t necessarily collect data in a way that makes it easy to geo-code. There would be notes like ‘This is grandma’s telephone line.’ I went line by line to clean it up.” He wound up with a list of three thousand individuals. His interest was recidivism: Were those people who came out of prison and found their entire world destroyed more likely or less likely to end up back in prison than those who could go home again? Kirk looked first at the results one year and three years after release and has since been working on an eight-year study. The results aren’t even close. Those who went home had a recidivism rate of sixty per cent. Those who couldn’t go home had a rate of forty-five per cent. They moved away. Their lives got better.

“This spring, I was on a radio talk show in Houston, Sunday morning,” Kirk said. “This guy was listening. He called me up. He is a crack addict, with multiple incarcerations for burglary and theft. This is a guy who grew up in Arkansas, didn’t have a very good childhood. He went down to Louisiana, and spent his entire adult life in New Orleans. Then he moved to Houston. I don’t know his exact age, maybe a fifty-year-old black male. And this is what he told me: ‘Now, I hate that the storm came because a lot of people died in the storm, but, guess what, that was probably the best thing that could have happened to a lot of people, because it gave them the opportunity to reinvent themselves if their life wasn’t going right.’ ”

New Orleans is a city framed by two major bodies of water. Its northern edge is Lake Pontchartrain, which empties into the Gulf of Mexico. Running through the southern part of the city is the Mississippi. Connecting the two—lake and river—is the Industrial Canal, built almost a century ago. It runs right down the middle of the Ninth Ward, separating the Upper Ninth, which includes the rapidly gentrifying neighborhoods of the Bywater, from the Lower Ninth, a smaller, isolated rectangle on the eastern edge of the city.

The most affluent parts of the city are on the river side: the Garden District, Uptown, the St. Charles and Magazine Street arteries. This is the historic New Orleans of grand houses, Audubon Park, and Tulane University. Richard Campanella, a geographer at the university, calls the area the “white teapot” of New Orleans—“teapot” for the way it follows the curving path of the Mississippi. As a delta plain, New Orleans has the curious effect of building uphill toward the river, so the teapot is on high ground. East and north of the teapot is Central City. It is almost all black. “Historically, the Central City population tends to be post-emancipation, urban migrants, coming in from the plantation regions, sugarcane fields, and the cotton fields in Mississippi and upriver,” Campanella said. “They were more likely to be Protestants, more likely to be ‘Anglo’ African-Americans—English-speaking, etc.” Campanella was in his office at Tulane, the heart of the white teapot. Central City was a ten-minute drive away, but it could have been across the ocean. Beyond Central City is Treme. The historical roots of Treme are much older—in the black Creole communities that date back to French and Spanish colonial times. “Many are descendants of the free-people-of-color population,” Campanella said. “Many of them were in the skilled trades, many of them professionals. They had a bit of access to the middle class.”

Using census data, Campanella has created a computerized map of New Orleans, in which the city is divided up into thousands of little squares—each representing a city block and each color-coded according to the predominant race of that specific area’s residents. He put the map up on his computer screen and pointed to the river side of the Lower Ninth. “Up until the nineteen-sixties, you would have seen mostly working-class to middle-class ethnic whites living here,” he said. “After school integration, we had white flight over here”—he highlighted St. Bernard’s Parish, directly across the city line—“at which point the Lower Ninth became about ninety-five per cent black.” He pointed to the Seventh and Eighth Wards. In the nineteen-sixties and seventies, blacks from neighborhoods like Treme moved upward toward the lake, to places like Pontchartrain Park and Gentilly. The whites who had been there moved to the suburbs. “There was a very rapid turnaround,” Campanella said. The lines between black and white hardened, and, for a generation, New Orleans was laid out like a checkerboard. If you were African-American and lived in Central City or Treme or the Lower Ninth, chances were that your parents lived there, too, and maybe even your grandparents—perhaps even in the same house.