Obesity Causes

From an interesting new article in Science, scientists attempt to answer why obese people are obese at all. They find evidence using MRI scans and a controlled experiment that to the obese person, food is not as satisfying as it is to the control group - measured by the amount of activity in an area of the brain with a lot of dopamine receptors. So, in order to feel full, they have to eat more, and therefore they become heavy.

It's an interesting, simply story, but I don't quite understand the link between food satisfaction (ie, the taste and the utility) and being full. But, I guess they're saying that the marginal utility of food consumption is diminishing at a lower rate for the obese person than it is for the non-obese person, in which case each additional bite is still yielding more utility. But then if that is what they're saying, that sounds like the opposite of the "less satisfaction" story. If food is actually yielding less satisfaction, then it seems like they'd stop eating sooner and substitute to something else.

I'd also like to point out the weakness in studies like this. Obesity is not exogenous. They didn't assign obesity to the control and treatment groups - individuals were selected into the study based on existing obesity. Thus it's entirely possible obesity itself causes the things they observed in the MRIs, rather than itself being a cause. That kind of reverse causation is not going to be easily dealt with in studies like this.

Another study once focused on MRIs between gay males and straight males, found differences, and concluded they had found evidence of something in the brain that caused homosexuality, but again, it's entirely possible that homosexuality caused the thing they observed. The same story applies here - obesity could be causing the thing they are observing. Kind of amazing when you think about it - that's such an obvious point and yet the article is in Science.