If it’s genetic, why is it changing?
| Gabriel |
In the course of an extended discussion of obesity (here, here, 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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