Today's Times reported on a new NBER working paper that finds increased obesity caused by the presence of fast food restaurants.
Currie et al. focus on 9th graders and moms, and what's great is that the effects of geographic proximity are indeed estimated stronger for 9th graders than for moms. You'd think that having fast food within your zip code, as opposed to just a little further away, might not matter a lot for folks with cars, and apparently that's somewhat true.
But if you're walking to the fast food restaurant, aren't you reversing some of the obesity effects? The issue is what's the treatment and control; if there are fewer fast food joints around, presumably those who would be walking to them would instead be walking to obtain healthier food alternatives in the counterfactual. But I wonder where 9th graders would otherwise be going than to fast food. Their counterfactual may be eating lunch at school, but then again, maybe 9th graders aren't allowed off campus and what we're really talking about is after-school eating and possibly dinners.
Looking at the study, you might also raise your eyebrows at the definition of what's fast food and why that matters. Are meals really healthier at IHOP and Applebee's than at McDonald's? But the story is more about calories per dollar, which is far higher at McDonald's.