The Variance of Fat

I was under the impression that the whole distribution of body weight was shifting to the right.

That is, the average person weighs more but there is an approximately constant distribution around the mean. This is wrong.

Check the following figure from a discussion of obesity trends by the Center for Disease Control:

Obesity has almost tripled since the early 1960s (13% to 35%). Extreme obesity has risen by more than a factor of 6 (0.9% to 6.2%). Yet mere overweight status, a BMI between 25 and 30, has remained stable at about 32% of the population. The action is in the tails!

For a more complete view of the distributions for 1976-80 and 2005-6, look at the density figure on page 2 of this data brief.

To me it looks like BMI is distributed log-normally and the sigma parameter has increased. I don't see how this would be predicted by either the biological or economic explanations discussed in my last post. Two ideas occur to me: First there is heterogeneity in the response parameter. As fat becomes cheaper, many are unaffected but some respond a great deal. Second, there are multiple equilibria. Given the opportunity to get fat people take it only when their neighbors do.