Sprawling Waistlines

When your city spreads out, so does your paunch – at least according to new work from the NBER stable. Their IV strategy seems credible, suggesting that the relationship is probably causal.

Effects of Urban Sprawl on Obesity (unstable ungated, stable gated)
Zhenxiang Zhao and Robert Kaestner

In this paper, we examine the effect of changes in population density–urban sprawl–between 1970 and 2000 on BMI and obesity of residents in metropolitan areas in the US. We address the possible endogeneity of population density by using a two-step instrumental variables approach. We exploit the plausibly exogenous variation in population density caused by the expansion of the U.S. Interstate Highway System, which largely followed the original 1947 plan for the Interstate Highway System. We find a negative association between population density and obesity and estimates are robust across a wide range of specifications. Estimates indicate that if the average metropolitan area had not experienced the decline in the proportion of population living in dense areas over the last 30 years, the rate of obesity would have been reduced by approximately 13%.

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2 Responses to Sprawling Waistlines

  1. Michael W says:

    Why should that hold? It might be about tranport, for 3 reasons: (1) if you live near work you are more likely to be able to walk, ride a bike or at least do some walking to and from public transport; (2) probably more importantly, if you are spending 2, 3 or 4 hours on your daily commute, you just don’t have time for any exercise. When could you do it? (3) if you are spending a long time travelling home you are so tired by the time you arrive that you don’t have the motivation to cook – you grab takeaway, or thaw out some junk food.

  2. Kevin Cox says:

    Or it could be that poorer people are being forced out of gentrified areas and perhaps poorer people have a high calorie low cost diet.

    This illustrates my concern about this type of reporting of such results. The title is “Effects of Urban Sprawl on Obesity”. Sorry it is not urban sprawl that is the cause. These sorts of studies can and do bring about policies such as we are experiencing in Gungahlin (an outer area of Canberra) where we have urban sprawl and high density with a medium sized block now being defined as 250 square meters. The reason it brings about these consequences is that such studies suggest that urban sprawl is bad and the only way to counter it is to have higher density and if we have higher density then we may as well start in the outskirts of cities because that is easiest to do because we are building those areas.

    I suggest that authors and commentators should be very very careful not to imply by their titles and comments that there is a causal effect between urban sprawl and waistline expansion as it can lead to unintended consequences. They should find the reasons why there is the correlation which may be the ideas suggested by Michael W. If it is lack of exercise because of travel time then a better solution than high density outer suburbs is low density outer suburbs with plenty of jobs and plenty of places to exercise and walk to work.

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