Despite numerous studies, economists don't have a great explanation for the obesity epidemic. But even though we don't know what exactly is the main driver behind the chunk-ification of America, we may be getting a better sense of what isn't responsible.

I wrote in June about one study which found no link between fast food consumption and obesity. Now, a new study finds no evidence that the proliferation of Wal-Mart stores, many of which are packed with cheap processed foods, has led to weight gains. In fact, the existence of a big box retailer in an area seems to actually decrease weight slightly.

Charles Courtemanche from University of North Carolina at Greensboro and Art Carden of Rhodes College in Tennessee looked at county-level data on big box stores (different versions of Wal-Mart stores as well as Costcos) and survey data which captured residents' demographic information as well as eating, shopping, smoking, drinking, and exercising habits. They found that one additional regular Wal-Mart store (which doesn't sell groceries) was associated with a drop of 0.5 pounds for a person of the average height. An additional Super Wal-Mart, which does sell groceries, was associated with a weight drop of 0.18 pounds.

The researchers think this happens because cheaper goods from Wal-Mart allow shoppers to spend more of their budget on relatively expensive healthier foods. Providing some support to their assertion, when Courtemanche and Carden broke down their results by income, they found that the lowest-earning people saw the most weight loss:

This is not surprising since low-income consumers would presumably be the most sensitive to modest changes in prices and purchasing power.

Courtemanche and Carden also found evidence that purchases of fruits and vegetables increased after the introduction of a big box retailer. But the news isn't all good: An additional Wal-Mart also led to decreased exercise, though the reasons for this are somewhat mysterious. The researchers suggest a couple of reasons:

...these stores may in fact reduce functional exercise by causing a shift from walking-intensive downtown shopping to driving-intensive suburban shopping. Alternatively, if people are losing weight due to reduced fat intake, they may feel less of a need to exercise.

Neither of these is very convincing though. Still, overall Courtemance and Carden's paper may mean that Wal-Mart's slogan -- "Save money. Live better." -- may have a pound of truth to it.