Identical twin studies show that obesity has a heritability of almost 80%. Although I generally lean towards nature in most nature-nurture debates, I’ve always had a problem with the idea that obesity is highly genetic, and thus enjoyed this epic rant by blogger Robert Lindsay:

It is 80% genetic[?]

That is why you have whole tribes in South America where not one person has ever been fat.

That is why you have whole towns in Melanesia with 1000’s of people where not one person is fat.

There are fat people in the cities of Solomon Islands. In the study I read, the only man who was fat was one who had gone off to the city for a while and ate salt and processed, packaged food. Do you realize that if you did a genetic study of the fatties in Melanesia, you would find that wonderful 80% “genetic” link you guys are shouting about?

That is why the fatness and obesity rate has exploded in the US and much of the rest of the world. Because it’s 80% genetic!

I do not believe that fatsos act just like the rest of us. Ever known a blimp who ate like a bird? Me either.

I dunno about you, but I have never seen a fat person who wasn’t stuffing their face all the time with lousy food. They are always in restaurants. Always going out to eat. If you go to a restaurant, look around at all the fat people. Those people are fat because fat people like to eat out all the time and restaurant food is fattening. Fat people love to eat. Have you ever noticed that?

This was Robert Lindsay at his best. Although he might not know the technical details of what 80% heritability means, his natural high IQ just shines right through in the form of common sense.

In fact just the other day, I was at the home of someone who was so incredibly fat I thought “it must be genetic.” And then just as I was leaving his house, I noticed a huge empty box of pizza in the kitchen.

Now I have no doubt that if that person has an identical twin raised apart, he too is extremely fat, and thus fatness technically has a high heritability, but what exactly is genetic here? The fatness itself or the tendency to engage in behaviors that cause fatness such as ordering large pizzas? So while obesity might technically be nearly 80% genetic, the statistic is misleading because it’s not directly genetic in the same was as height is.

It is interesting to ask (and commenter Mugabe/Videla has alluded to this) whether the high heritability of IQ is qualitatively more like height (directly genetic) or like weight (largely indirectly genetic). If I had to guess I would say that the heritability of fluid IQ is more like height, while the heritability of crystallized IQ is more like weight, but that’s just my opinion.

Of course I don’t deny obesity has some direct genetic component. Some people gain weight a lot easier than others and for some people, it’s virtually impossible to lose weight no matter how well they eat, though this is rare.

It’s also true that most people who lose weight end up gaining it back, but that’s because they end up returning to their compulsive eating habits.