To emphasize, I do not actually believe that assortative mating is the cause of the obesity epidemic.

A major challenge to any theory, however, is that it has to operate as a multiplier. The heritability of obesity is seemingly constant. That is, what we mean when we say “Whatever is happening, its seems to be happening to all of us”

If that were not true then presumably some people would be protected not only by their genes but by their environment. In which case heritability would fall.

For example, suppose that it is fast food. Well some people may be adopted into families which don’t eat fast food. This person should have a large degree of protection that is not genetic. Yet, the data don’t bear that out. Environment has not become more important.

Suppose it was watching TV, or drinking Soda, or any of those things. Then some sort of environmental protection should show up in the data but it doesn’t seem to.

This is why I have argued before that I suspect it is a ubiquitous small molecule. Its something that’s in everything. Maybe a universal preservative. Or, a legally mandated fire retardant. Something like that.

Some of the people I am fans of suggest that it is sweeteners like High Fructose Corn Syrup. This is not nutty as there is a clear mechanism and HFCS is more universal than you might think. However, its still a big hurdle. Surely some parents restrict their kids exposure to HFCS. Why don’t we see that?

Further my point is that a wholly genetic theory of the obesity epidemic can be constructed and to my knowledge no one has laid down solid evidence that it is in fact wrong.

That’s different from thinking its right, but it does highlight the state of our ignorance.

Also, for there are a couple of go-to studies on Obesity and Heredity. I’ll pull a chart from one of my favorite hereditary studies, the Sacerdote’s “What Happens When We Randomly Assign Children to Families” because this is the type of study we are typically tying to emulate statistically.


This shows the relationship between the child’s outcome and that of their adoptive parents and their biological parents. The children are on average about 30 or so I think.

Some outcomes are of course going to be influenced by neither. However, you can get a sense of the relative contribution of “role model” vs “genetic” effects by looking at the ratio.

On this measure BMI and Height are about the same. Studies usually find height is about 90% genetic and BMI is between 70% and 90% genetic. Usually measures of obesity as a binary variable find lower genetic effects but because “unexplained” variation is categorized as environmental this is at least partially and possibly entirely due to binary cut-off.

That is, if you have a BMI of 29.9 and your twin has a BMI of 30.1 then you are obese and he is not. However, this would look like unexplained variation when really the variation is an artifact of the cut-off.

Lastly, you can build a model that says its just self-control. One’s self-control is genetic and what has changed is the opportunities for indulgence. Maybe, but its hard to square that with childhood obesity which is under control of the parents and thus we should see a strong affect of strict adoptive parents.

There are also some other issues with self-control being the mechanism that I’ll just mention. One, self-control seems to be rising by almost all other measures yet obesity is rising. Not a killer problem for the theory but perplexing.

More importantly its hard to square with the stability of weight, which is probably the biggest or second biggest next to heritability, stylized fact about obesity. That is, if you always over eat you don’t get fat, you get fatter. Presumably, you will get fatter and fatter over time.

However, non-dieting obese folks have a weight stability that is roughly the same as thin people. Its as if they tried to go exactly to a particular too heavy weight and then stop and act like a thin person with 50 extra lbs.

Why does it stop like that?

That’s a puzzle.

It also *suggests* that there is some regulating mechanism that is set to the wrong parameter. We can also measure regulating hormones and they are systematically off in obese people, but this is enough for one post.