The (traffic jammed) Road to Fatness?

Commenting on an Yglesias post connecting obesity and commute times, Karl Smith asks for a model to explain the effect:

I tend to roll my eyes at this type of research because in the language of economics, it lacks a structural model. That is at model time folks seem to forget that they are dealing with a complex system that has specifically evolved to maintain equilibrium in the face of exogenous shocks.
Moreover, the human system is not much different than most mammalian systems and so positing cognitive causes as the source seems sketchy at best. This is not to say impossible, but simply that a claim so ridiculous on its face would require a really good story. To date I have yet to hear one and typically when I’ve challenged obesity researchers with an implicitly cognitive model they typially seem unaware that one would need a story or structural model.
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We’ve observed that certain kinds of chronic stress can have strange effects and we know that we can manipulate body mass by using drugs that manipulate serotonin, dopamine,norepinephrine and cortisol. So, we have something remotely suggestive of a mechanism.

All that having been said my intuition still says we are looking at a single vector, and it’s a molecule or family of molecules. Still, something like the commuting/sitting hypothesis should be taken seriously.

It sounds like Smith wants to hear something along the lines of “commuting is known to cause an increase in levels of the whatever molecule, and the whatever molecule is known to up/down regulate the whatever pathway, leading to weight gain”.

That would be fantastic if we could figure it out to that level of detail but its not going to happen. Rather, I think the effect is much, much more simply explained by time constraints. Let’s say you work 8 hours per day plus 1 hour for lunch and breaks. Now let’s say you commute 1 hour each way (a conservative estimate for many) and take half an hour in the morning to get ready. This means from the time you wake up to the time you get home from work consumes 11.5 hours. Now let’s say you want 6.5 hours of sleep each night. Were up to 18 hours accounted for. That leaves 6 hours in which to run errands, do chores, cook and eat dinner, prepare for the next day, go out and drink, or do whatever else you may want to do. So with only 25% of each day available to you to do all the things you want/have to do, activities like going to the gym or exercising probably won’t make the cut for most people.
Of course that’s a choice and anyone that so chooses could prioritize those activities over happy hour for example. Commuting certainly doesn’t make you fat, just like being poor doesn’t make you fat. It’s all about choices.

But time is a very real constraint, and should factor into the analysis. At the end of the day you can still choose to eat healthy and exercise, regardless of your commute (or income) but the associated time constraints serve to raise (potentially significantly) the marginal cost of making those choices.

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