If it’s genetic, why is it changing?

August 3, 2009 at 4:38 pm 2 comments

| Gabriel |

In the course of an extended discussion of obesity (herehere, here, and here), Megan McArdle mentions that weight is highly heritable. She doesn’t mention it, but it’s also true that the psychometric latent variable “g” (aka, IQ) is highly heritable. The puzzling thing is that both obesity and IQ have been increasing over the last few generations. This would make sense if there were selective mortality and/or fertility such that intelligent fat people were more likely to live and have children than stupid skinny people, but there is no such selective pressure to any appreciable degree. So here we have the apparent paradox of a rapidly changing phenotype of a highly heritable trait despite minimal change in the genotype.

The paradox comes from (implicitly) understanding heritability as meaning something like a regression coefficient when it’s really more like a correlation coefficient (technically it’s a structural equation modeling phi). Although we think of correlations as being even more basic than regression coefficients, the interpretation is actually weirder. GNXP provides a very good explanation of this that is well worth reading in its entirety, but here’s the take home:

To say that a trait is .95 heritable does not mean that it is caused 95% by genes, that’s not even wrong. Rather, it is to say that 95% of the variance within the population can be accounted for by the variance of genes within the population. But heritable traits are also usually affected by environment; if you starve someone they will be short, but retain five fingers. The number of fingers you have on your hand is not heritable, because there’s no real variance within the population of the trait. It’s genetically specified, but not heritable.

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2 Comments

  • 1. Rachel  |  August 13, 2009 at 3:57 pm

    Hi Gabriel. Ack. I didn’t take stat. But I just found your writing and it’s compelling me to get more familiar with some of these basic statistical concepts. Can you recommend a really basic resource that I could read to get familiar with the terms – things like regression and correlation coefficient?
    Also, particular to obesity and diseases of obesity (heart, diabetes) have you read the Jungle Effect by the physician Daphne Miller? She looks at several specific “cold spots” with longevity & good health, but all seemingly contradictory diets (high veggies and low meat in Okinawa, lotsa fat and little veggies in iceland…but everyone healthy).

    • 2. gabrielrossman  |  August 13, 2009 at 4:26 pm

      hi rachel,
      i think the simplest way to get familiar with the gist of basic statistical concepts is The Tao of Statistics by Keller. Real quick though, it’s easiest to think of regression in terms of a scatterplot, where each case (eg, a survey respondent) is a dot whose location is defined as x and y. Imagine having a cloud of a few hundred of these dots, then drawing a line through the center of the cloud. The regression coefficient describes the slope of this line and the correlation coefficient describes how close to the line most of the dots are. So a regression coefficient describes the nature of a relationship and a correlation coefficient describes the strength of a relationship.

      i’m only casually familiar with the peculiarity that wildly divergent diets seem to work, all i can say is that it certainly seems weird. (fortunately i’m not a dietician so i have the luxury of not taking an opinion on this).

      also, seeing that you have a foodie blog you might enjoy this article on gourmet magazines


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