The “average life expectancy” of a city is a statistically curious number. It’s not really a prediction about how long you’re going to live. It’s an average of how long everyone here lives — and thus it forms a good barometer of the overall health of the city. In particular, a city’s average life span is sensitive to the rates at which people die too young. Since the average New York life expectancy is now 78.6 years, anytime someone dies younger than that, it drags the city’s overall average down slightly.

The math works like this. Imagine that one man dies of AIDS at age 25. Since he was statistically supposed to live to 78.6 years, he’s died about 50 years too early, so he shaves 50 years off the city’s overall pool of life. If one Wall Street guy collapses of a heart attack at age 65, he shaves only ten years off. You’d have to have five Wall Streeters die at that age to equal the impact of one AIDS victim. By the same logic, one infant’s dying during childbirth — 77.8 years too early — is equal to ten people’s succumbing to lung cancer at age 70. It is a very weird form of horse trading. The more you’re able to prevent young people — folks in their twenties and thirties — from dying, the more rapidly you boost a city’s overall life expectancy.

And this is precisely what the city has done, through a combination of smart public policy and sheer luck. All the boons of the nineties — the aggressive policing, the dramatic drop in crime, the renaissance of the city’s parks and street life, the freakish infusion of boom-time wealth — played a part. Take the miraculous evaporation of the homicide rate. In 1990, a stunning 2,272 New Yorkers were murdered; in 2005, that number dropped to 579. Since a majority of those being killed were younger men, the reduced murder rate alone added tens of thousands of years to New York’s life-expectancy pool. Another big drop was in HIV mortality rates. In 1994, deaths from AIDS peaked at over 7,100, but the arrival of better drugs and health care began to whittle that number by 80 percent — so in 2005, only 1,419 died of AIDS. Again, the majority of the lives saved here were those of younger men, resulting in a disproportionately big upward leap in our city’s life span. In 1989, the infant-mortality rate was 13.3 babies per 1,000, and by 2004, it had been halved, to 6.1, both because medical treatment improved and because alcohol and drug addictions eased. To top it off, drug-related deaths, another arena with disproportionately younger victims, tapered off, too.

Homicide, AIDS, and drugs are characteristically New York ways to die young, of course, so it’s no surprise that when we sharply decreased the fatalities they caused, we caught up with the rest of the country. But here’s the thing: It’s not just that we’ve conquered these urban blights. Cancer and cardiac arrest are down, too. The number of people in the city dying from heart disease has dropped by a third in the last twenty years, and cancer rates have slid by nearly a fifth. And again in these cases, New York is getting healthier faster than the rest of the U.S.

In essence, there is a health gap emerging between our massive metropolis and the rest of the country — some X factor that’s improving our health in subtle, everyday ways. In fact, a back-of-the-envelope calculation shows that once you take out those uniquely New York ways to die — AIDS, homicide, etc. — we’ve still added at least 200,000 extra years onto the city’s life-expectancy tables since 1980, making crucial advances in the same health areas the rest of the country struggles with. Like many New Yorkers, I’d moved here with some trepidation — always figuring that the stress, pollution, and 60-hour workweeks would knock about five years off my life. I was wrong — precisely wrong. But where, exactly, is our excess life coming from?

I TAKE THIS QUESTION TO THOMAS FRIEDEN, New York’s commissioner of public health. Frieden is a wonk’s wonk — a handsome, energetic doctor who has gained a nationwide reputation for his aggressive effort to push New York’s average-life-expectancy figure ever higher. The smoking ban of 2003? The trans-fat ban of last year? You can thank Frieden for both. These measures have already begun to lengthen life spans in the city. The smoking ban had an immediate effect: The number of deaths attributable to smoking has decreased from 8,960 in 2001 to 8,096 in 2005, a drop of 10 percent. Lung-cancer rates should begin to see the same effect a few decades from now, since it takes longer for the body to repair smoking-related lung damage.

But even Frieden admits that public policy can’t account for all the gains. When I ask what the X factor is — where the “excess life” is coming from — Frieden goes over to his desk and returns with a clear plastic statuette. It’s from the American Podiatric Medical Association and Prevention magazine: BEST WALKING CITY, 2006.

“We’ve won it a couple of years in a row,” he tells me with a grin. He’s got a bunch of them kicking around.

Walking? This isn’t quite as facile an explanation as it sounds. Scientists who study urban health argue that it’s not just that we walk more — it’s the way we walk that has a surprising spillover effect on life spans. Researchers have long known that people here walked fast — far faster than anyone else in the country. Indeed, the easiest way to tell a New Yorker from an out-of-towner is by walking speed: The natives blast down the sidewalk at blitzkrieg pace, and the visitors mosey along like pack mules. Eleanor Simonsick, a Baltimore-based epidemiologist, knew that regular walking is a powerful way to maintain your health. But she began to wonder, a question very germane to us in New York: Does the speed at which you walk also affect your health?

She decided to conduct an experiment to find out. She and a group of scientists assembled 3,075 seniors in their seventies and asked them to traverse a 400-meter course, walking as fast as they could. They monitored their subjects’ health over the next six years, during which time 430 of the geriatrics died and many more fell ill. When Simonsick crunched the data, she found that the ones who were dying and getting sick were the ones who walked the slowest. For every minute longer it took someone to complete the 400-meter walk, he had a 29 percent higher chance of mortality and a 52 percent greater chance of being disabled. People who walk faster live longer — and enjoy better health in their later years.

“Walking speed absolutely reflects health status,” Simonsick says. So when you irritatedly blow past a trio of ambling visitors from Ohio or Iowa on the subway platform, you’re not just being an obnoxious New Yorker. You’re demonstrating that you’re going to outlive them — and enjoy better health while they slowly degrade.

The thing is, as Simonsick points out, New York is literally designed to force people to walk, to climb stairs — and to do it quickly. Driving in the city is maddening, pushing us onto the sidewalks and up and down the stairs to the subways. What’s more, our social contract dictates that you should move your ass when you’re on the sidewalk, so as not to annoy your fellow walkers. (A recent ranking of cities found that New York has the fastest pedestrians in the country.) As Simonsick sees it, the very structure of the city coerces us to exercise far more than people elsewhere in the U.S., in a way that is strongly correlated with a far-better life expectancy. Every city block doubles as a racewalking track, every subway station, a StairMaster. Seen this way, the whole city looks like a massive exercise machine dedicated to improving our health while we run errands.

This idea of the city as a health club is fairly revolutionary. Back in the beginning of the industrial revolution, cities were regarded, quite correctly, as lethal places to live. London and other newly ballooning industrial centers did not yet have sanitary or pollution laws, and the sudden influx of crammed-together citizens — living cheek by jowl with smoke-belching factories downtown — produced spectacular outbreaks of disease. Public-health experts somberly wrote about an “urban health penalty” — the idea that cities were dark, satanic mills that inherently cut us down in the prime of life. In the first decades of the twentieth century, cities began to clean up their acts drastically, when sanitation standards emerged and inoculations began to aggressively squelch infectious diseases; the actual life spans in cities began to catch up to and exceed those of people in rural areas. But the idea of urban rot remained strong, so the cultural bias against urban life lived on. It didn’t help when the seventies and eighties ushered in waves of urban crime, recession, and drug epidemics, and cities like New York and Detroit and Chicago sharply curtailed public-health services. Cities, more than ever, seemed like cesspools of dread and early death.

By 2000, though, the perspective looked altogether different. With a sharply reduced crime rate, runaway gentrification, and a geyser of boom-time dough, Manhattan had largely conquered the homicide, AIDS, and overdose problems that were pulling down the average life-expectancy figure. A trio of New York–based urban-health academics — Nicholas Freudenberg, David Vlahov, and Sandro Galea, professors at Hunter College and the New York Academy of Medicine — began to wonder if the “urban health penalty” still made sense. As they examined the most recent data about health in cities versus health in rural and suburban areas, they noticed that the cities were, contrary to theory, pulling ahead. This wasn’t merely because cities tend to have richer citizens. In fact, they found that people are almost equally likely to be poor — and to lack health insurance — in urban and rural areas. Yet the percentage of rural people who ranked their health as “fair/poor” is much higher than in urban areas. And people are more likely to die young in the sticks: Death rates for 1-to-24-year-old males are 60 per 100,000 in cities, versus 80 in rural areas. Perhaps worst of all for the suburbs, obesity is rising far more rapidly than in cities.

“We were just walking around New York and thinking, Wait a minute,” Vlahov says. “People in New York are in better shape than ever. So there’s obviously got to be something about cities that’s good for you.”

The urban health penalty, they decided, had inverted itself. The new reality was that living in the suburbs and the country was the killer. In January 2005, Vlahov and his colleagues penned a manifesto they cleverly called “The Urban Health ‘Advantage,’” and published it in the Journal of Urban Health. Cities, they posited, were now the healthiest places of all, because their environment conferred subtle advantages — and guided its citizens, often quite unconsciously, to adopt healthier behaviors.

THREE YEARS AGO, LAWRENCE FRANK, a professor of urban planning at the University of British Columbia, set out to measure this effect, examining 10,858 people in Atlanta and the type of neighborhood they lived in. Some were in purely residential suburban neighborhoods, where you had to get in your car to buy a carton of milk; others lived in “mixed” downtown areas with shops within walking distance. When he checked the results, the health difference was shockingly large: A white man who lived in a more urban, mixed-use area was fully ten pounds lighter than a demographically identical guy who lived in a sprawling suburb.

“The more you drive, the more you weigh,” Frank tells me after I call him to talk about it. He was unsurprised when I described New York’s increases in life expectancy. “You put people in an environment where public transportation is rational and driving is almost impossible, and it would be shocking not to see this outcome,” he says. Other scientists suggest that New York’s benefits do not occur merely because the city is walkable. It’s also because New York is old and filled with attractive architecture and interesting street scenes — since, as it turns out, aesthetically pretty places lure people out of their homes and cars. A 2002 study by the National Institutes of Health found that people living in buildings built before 1973 were significantly more likely to walk one-mile distances than those living in areas with newer architecture — because their environments were less architecturally ugly.

At the same time, New Yorkers are also more likely to visit parks than people who live in sprawl, because the parks are closer at hand. And proximity matters, as a study by Deborah Cohen, a senior natural scientist at the rand Corporation, discovered. When she examined the use of several parks in Los Angeles, she found that almost half the people using any given park lived no more than a quarter-mile away. In contrast, only 13 percent of the people using the park had come from more than a mile away. “The farther you are, the less willing you are to go to the park,” she notes.

Interestingly, urban theorists believe it is not just the tightly packed nature of the city but also its social and economic density that has life-giving properties. When you’re jammed, sardinelike, up against your neighbors, it’s not hard to find a community of people who support you — friends or ethnic peers — and this strongly correlates with better health and a longer life. Then there are economies of scale: A big city has bigger hospitals that can afford better equipment — the future of medicine arrives here first. We also tend to enjoy healthier food options, since demanding foodies (vegetarians and the like) are aggregated in one place, making it a mecca for farm-fresh produce and top-quality fish, chicken, and beef. There’s also a richer cultural scene than in a small town, which helps keep people out and about and thus mentally stimulated.

Of course, the built environment wouldn’t have done New Yorkers’ health any good if it hadn’t been catalyzed by the city’s economic bonanza. The nineties were so lush they actually lifted some of the city’s poorest out of poverty. But gentrification cut both ways. A more cynical — and possibly clear-eyed — explanation for New York’s life-expectancy gains is that gentrification drove many of the city’s poorest people out of town. Though no figures exist to accurately calculate it, what social scientists can measure is the effect of gentrification on the health of the poor who have stayed put: It turns out to be — unexpectedly — benevolent. One study by Ming Wen, a sociologist at the University of Utah, crunched data on 8,782 residents of various neighborhoods in Chicago. She expected to find the typical bleeding-heart conclusion: Poverty is bad, income inequality is bad, and the two together are worse yet. But in reality, income inequality at the neighborhood level paradoxically seemed to mitigate the bad effects of poverty. In neighborhoods that mixed affluent people alongside poor ones, the poorer residents were statistically healthier than those in non-mixed neighborhoods.

That’s because, Wen concluded, the presence of relatively wealthy people has a spillover effect on the immediate neighborhood: safer streets, cleaner environments, better food in stores. (Indeed, another study found that poor teenagers in mixed-income neighborhoods ate more leafy green vegetables than poor teenagers in non-mixed ones.) Wen is careful not to say that all income equality produces trickle-down effects; if the poor and wealthy are completely sealed off from each other in different parts of a city, the effect doesn’t occur.

AND THIS, AS IT TURNS OUT, helps explain the one troubling chapter in New York’s life-expectancy success story: the Bronx. Alone among the five boroughs, the Bronx’s average life expectancy has actually declined in the last twenty years. And it is the only one that saw very little financial uptick from the nineties boom years, and virtually no gentrification.

The effect on everyday health becomes pretty apparent to me when I take a trip up to St. Barnabas, the Bronx’s largest acute-care hospital, to meet with Jerry Balentine and David Perlstein, the chief medical officer and associate medical director, respectively. They urge me to wander around a bit and look at the local bodegas, where the food options are pretty lousy — mostly fatty canned foods and virtually no fresh vegetables. The new reality is, the Bronx is ballooning. “You walk along here and you almost never see an actual supermarket,” Balentine says with a shrug. “So people can’t eat healthily even if they want to. It’s all fast food. That’s what’s cheap — Chinese food, pizza.”

Perlstein takes me for a stroll through one of St. Barnabas’ clinics, and it’s hardly a picture of good health. Virtually everyone is overweight, many enormously so: One white-haired woman poured over the edges of a small chair as she sat knitting; she looked as though she could easily crack 300 pounds. “This is our biggest problem,” he says. There’s an ethnic component; Hispanics tend to be stockier to begin with, he notes. But there’s also a cognitive drift among his patients. Since they’re surrounded all day long by people who are huge, they lose the ability to recognize what it means to be overweight. People who are healthy look creepily skinny.

“I get mothers coming in with their kids, and the kids are already looking a little too heavy, right?” Perlstein says. “But the mothers are going, ‘He’s not gaining enough weight! Give me a pill that makes him gain more weight!’ They see being heavy as being healthy — you’re growing. It’s completely the opposite of what people think in Manhattan.”

Granted, New York is pushing various policies to affect the “food environment” in poorer areas of the city. In a pilot program in central Brooklyn, the South Bronx, and Harlem last winter, the city subsidized bodegas to carry one percent milk in addition to the unhealthier full-fat variant. And Frieden recently passed a law that will require most fast-food chains to prominently post the calorie content of their meals. But you can’t get past the sheer difficulties of being broke. Perlstein has female patients who schedule mammograms but then skip them — “because they’ve got three kids, and who’s going to look after them while they’re getting screened?”

AT TIMES, TALKING TO FRIEDEN and some of the other scientists, I wondered if all the talk about how healthy cities had become might be the latest species of boosterism, of civic mythmaking, partly because he’s staked his legacy on such aggressive policies as bad-food bans. And urban theorists have begun a fierce beat-down on the suburbs, castigating them endlessly for being the epicenter of the obesity epidemic. As it happens, this is the argument of Matthew Turner, an economist at the University of Toronto. Last year, he decided he was a bit sick of hearing about the health benefits of cities. The “urban health advantage” sounded to him like mere self-congratulation — the skinny, attractive folks in the megalopolises crowing about their innate superiority, and recoiling at the barbarisms of the SUV-driving, Wal-Mart-shopping exurban masses. It seemed too much like blue-state snobbery. So Turner devised a new experiment to test the power of the urban health advantage.

If it’s true that cities impose inherently healthier behavior on you, Turner reasoned, then people who move from cities to suburbs should get fatter — and vice versa. He began hoovering up data on 6,000 young Americans in their twenties to forties, tracking where they lived over a six-year period. He used satellite imagery and tallies of shops and churches to determine the level of sprawl in each subject’s neighborhood, then gathered information on each one’s weight.

When he examined the data, he discovered something surprising: People who moved between dense and sprawling neighborhoods didn’t change weight. Despite the claims of the new urbanists, Turner saw no evidence that one’s built environment has an impact on one’s health. “This idea that the built environment affects how much you weigh,” he told me, “is just wrong.”

But then why do cities harbor slimmer people who live longer and healthier than those in sprawl? Because, Turner argues, the populations are self-selecting. Highly active people who don’t like to drive — and who crave to make boatloads of money — naturally gravitate to places like New York, because that suits their chosen lifestyle. If we walk a lot here, it’s because we’re drawn to cities that force us to do so. The converse is also true: People who are heavier and less fit gravitate to suburbs precisely because that’s where they won’t need to walk — where nothing is possible without getting in a car. (Mind you, Turner’s rival scientists are not convinced by his argument. As one pointed out to me, moving to a differently dense area might take years to change your weight — longer than Turner’s time frame.) In Turner’s view, the logic of the urban health advantage is not only wrong, it’s backward. It’s not that New York makes us healthier. We make it healthier, by flocking here to live.

Ultimately, I’ve come to believe that Turner is likely correct — but so are the proponents of the urban health advantage. The two theories are not mutually exclusive. A city can be good for your health and, at the same time, attract healthy people.

The life-span miracle in New York — the X factor I’ve been searching for — is not one single cause, but a feedback loop. There’s no doubt that when New York cleaned up its crime in the nineties, it coasted to health as well as wealth in the high-tech boom: Wealthier people always live longer. But prosperity also wrought a cultural shift. New York once again became the city for young, ambitious strivers — precisely the sort that demand the cutting edge of healthy-living perks: an organic-food store on every corner, a yoga studio down the block, unreasonable amounts of sushi, clean parks in which to jog.

In a sense, the life-expectancy revolution challenges one of New Yorkers’ longest-held and oddly cherished self-mythologies: that our kinetic, aggressive city is a grim physical challenge, and we’re the Darwinian winners of the American race merely for surviving the mean streets. The truth is that the dystopic, self-destructive seventies are as gone as the days of the Bowery Boys.

Health has become our new urban stereotype. If New York City were still a raw, ungovernable failure, Frieden’s invasive nanny-state laws like banning smoking in bars or trans-fats would have made him a laughingstock. But the new New York has come to expect such measures; we probably even take a masochistic joy in being forced to behave ourselves. Hey, we’d been looking for an excuse to quit smoking anyway! And trans-fats — well, everyone knows that stuff’ll kill you, right? This is why Frieden will likely have no problem slapping another 50 cents onto the taxes for a pack of cigarettes this fall, either.

The urban health advantage is here, all right. And it is us.