Exercise from walking may have something to do with, but I suspect incentives from strangers have more influence on appearance.

I’m a little disappointed in you – this is, at base, an empirical question, and here you are proceeding with a largely-anecdotal debate. What happened to Felix the empiricist?

There’s a robust academic literature on this topic. Studies in New South Wales, California, and Atlanta suggest a fairly robust association. On the other hand, a study from Chicago and the results of a large-scale longitudinal survey cast doubt on the notion that there’s more than a weak correlation. The latter studies are particularly interesting. The longitudinal study looked at people moving from one environment to another, and found no substantial change in their weight, suggesting that we may be witnessing a sorting effect, rather than a causative one. And the Chicago study suggested a ring-model, in which the urban core was obese, the inner-ring affluent suburbs thin, and the outer suburbs only somewhat obese.

I’d suggest a way of integrating all of this data. To a substantial extent, patterns of development in the contemporary United States are strongly linked to other socio-demographic factors. Inner cities tend to be fairly impoverished; the close-in suburbs were built fairly early on, and tend to offer nice housing, high-quality schools, and shorter commutes; and the outer-ring suburbs tend to offer more cookie-cutter developments, longer commutes, but lower prices. It’s the classic donut-hole model of American cities.

The exception to this lies in the urban renaissance of the last decade and a half, as young professionals have flocked to select urban areas. In these segments of cities, the beneficial effects of urban living – lots of walking, in particular – are combined with the beneficial effects of affluence – a focus on health, sufficient leisure to exercise, foodie culture, and thinness as a marker of social class. That’s what you’re witnessing in Manhattan. But for most residents of urban areas, the benefits of walking continue to be more than offset by a far more substantial hurdle – the higher price of eating well, and the difficulty of finding fresh foods in inner cities. Without access to private transportation, most of America’s urban poor is stuck shopping at bodegas, inner-city markets, and fast-food joints, at which fresh fruits and vegetables and other healthful options are both scarce and expensive. Moreover, cramped quarters make it difficult to stockpile healthful options; most convenience food in these neighborhoods is high in fat and calories.

The upshot is this. If you’re affluent, you’re likely to be thin; if you’re poor, you’re likely to weigh more. The best of all worlds is to be affluent while living a non-sedentary lifestyle. The worst, as it turns out, is to be poor without having access to affordable healthy options for your diet. (The poor living in the outer suburbs actually have lower rates of obesity, despite their relatively more sedentary lifestyles, because they can actually buy good, cheap food.)

There are indeed lessons here. The most important is one that the Bloomberg administration recently announced it would pursue – using zoning to open up more full-service supermarkets in poor neighborhoods of the city. There’s probably no simpler way to increase the health of residents. But it’s also the case that DeBord is positively full of crap – driving is bad for you, end of story. Increasing urban density – when those urban areas also offer cheap and convenient access to fresh foods – is absolutely a net positive. That DeBord provides anecdotes of people staying thin in the outer suburbs simply demonstrates his innumeracy, and is a close cousin of the stories about Uncle Cletus living to 105 despite smoking a pack a day and washing down his dinner with rotgut whiskey. Sure, he did – but I don’t like the odds for me duplicating his feat.