A few friends have pinged me on this piece in Slate, Double Inanity: Twin studies are pretty much useless. The headline is bold, but the piece is just a sloppy mishmash. It’s really something amenable for a major “fisking,” but I generally don’t like doing that sort of thing, because it doesn’t seem a optimal allocation of time (though I have to note that the author seems to be implicitly using a colloquial form of the concept of heritability, which I think is going to confuse an already naive audience). A lot of the article is taken up with criticisms of political scientist John Alford’s behavior genetic findings on the heritability of ideology. I’ve had personal communication with other researchers in this area who actually are broadly critical of Alford’s methodology, but they’re still strongly invested in using genomics and behavior genetics to explore this issue. In other words, rejecting Alford’s conclusions does not entail that you just reject twin studies, as such.
Of course there’s a lot more to twin studies than behavior genetics. Luke Jostins at Genomes Unzipped had a long post up defending the methodology, with caveats, in estimating heritability. But what I find perplexing is that in many ways we are right now confirming the broad results of twin studies using genomic methods. I blogged a paper on the heritability of I.Q. a few weeks ago, but here’s a paper from 2006, Assumption-Free Estimation of Heritability from Genome-Wide Identity-by-Descent Sharing between Full Siblings:
…. Following the classic paper by R. A. Fisher in 1918, the estimation of additive and dominance genetic variance and heritability in populations is based upon the expected proportion of genes shared between different types of relatives, and explicit, often controversial and untestable models of genetic and non-genetic causes of family resemblance. With genome-wide coverage of genetic markers it is now possible to estimate such parameters solely within families using the actual degree of identity-by-descent sharing between relatives. Using genome scans on 4,401 quasi-independent sib pairs of which 3,375 pairs had phenotypes, we estimated the heritability of height from empirical genome-wide identity-by-descent sharing, which varied from 0.374 to 0.617 (mean 0.498, standard deviation 0.036). The variance in identity-by-descent sharing per chromosome and per genome was consistent with theory. The maximum likelihood estimate of the heritability for height was 0.80 with no evidence for non-genetic causes of sib resemblance, consistent with results from independent twin and family studies but using an entirely separate source of information. Our application shows that it is feasible to estimate genetic variance solely from within-family segregation and provides an independent validation of previously untestable assumptions….
The point is that before modern genomics one had to assume relatedness, and from these assumptions derived various estimates as to the heritability of a trait when taking into account patterns of genetic relatedness and phenotypic similarity. Today genetic relatedness can be pinned down on an individual basis with much greater precision. Two of my siblings for example have a relatedness of 0.42, instead of 0.50. This does not mean that all of the criticisms in the Slate piece are invalid, but there was a slapdash throw-the-kitchen-sink feel to it all.
Finally, I want to point out that just because a trait is not substantially heritable, it does not mean it is not substantially biological. This is the possible case with male homosexuality. I think as a scientific matter this is a huge issue, but when it comes to public policy it is much less important because the relevance of the genetic origin of a trait is that that necessarily means that it is biologically specified. If the biological specification is through random developmental processes or infection with pathogens, it means that it isn’t heritable and genetic, but it doesn’t mean that once the trait has fixed it is any less plastic.