Twin Studies and Beyond

Brian Palmer has a very weak article in Slate trying to make the case that “Twin studies are pretty much useless.” The article is supposed to be about the problem of twin studies as a method but it begins by raising the specter of eugenics. As if that were not enough guilt by association, Palmer then argues that twin studies threaten democracy or at least they would if they were true. (The argument is unclear but seems to rest on the false assumption that if genetics matters then nothing else does. Need I quote the tiresome point that poor eyesight has high heritability but that doesn’t make eyeglasses useless etc.)

After having muddied the waters, the author’s primary argument is this:

Twin studies rest on two fundamental assumptions: 1) Monozygotic twins are genetically identical, and 2) the world treats monozygotic and dizygotic twins equivalently (the so-called “equal environments assumption”). The first is demonstrably and absolutely untrue, while the second has never been proven.

On the first point, the fundamental assumption is not that MZ twins are identical but that they are more identical than fraternal twins. The math is a bit easier if you assume that MZ twins share all of their genes and fraternal twins share 50% on average but this is not necessary. In fact, if you take into account that MZ twins differ genetically this raises the variation that you should ascribe to genetics. If twin one smokes and twin two does not and you assume that they share 100% of their genes then you must conclude that smoking does not vary with genes. If the twins share only 99.99% of their genes then smoking may vary with genes.

On the second point (the equal-environments assumption), Palmer writes as if comparing MZ and DZ twins was the only source of heritability estimates. In fact, heritability estimates are found by looking at twins raised together and twins raised apart, siblings and siblings raised apart, parents and child correlations and so forth and the results from these studies are broadly similar.

Even more important, for an article that goes on about “modern genetics” the author seems completely unaware that it is now possible to do a whole-genome analysis. That is, instead of assuming that siblings share 50% of their genes on average it is possible to estimate, sibling-pair by sibling-pair, how many genes siblings share and then correlate that with various characteristics. Obviously, it takes a lot more data to do a study like this but it has been done. Visscher et al., for example, use data from 3,375 sibling pairs to estimate the heritability of height. Interestingly, they find a heritability of 0.8, very close to that found in traditional studies.

Using whole-genome methods it is not necessary to assume equal environments for MZ and DZ twins. In fact, using these methods you can do genetic studies across unrelated individuals. For example, in Genome-wide association studies establish that human intelligence is highly heritable and polygenic, the authors note:

Data from twin and family studies are consistent with a high heritability of intelligence, but this inference has been controversial. We conducted a genome-wide analysis of 3511 unrelated adults with data on 549 692 single nucleotide polymorphisms (SNPs) and detailed phenotypes on cognitive traits. We estimate that 40% of the variation in crystallized-type intelligence and 51% of the variation in fluid-type intelligence between individuals is accounted for by linkage disequilibrium between genotyped common SNP markers and unknown causal variants. These estimates provide lower bounds for the narrow-sense heritability of the traits.

Twin studies have their problems, just like any method. The thrust of recent advances–advances which have been made to analyze and surmount the kinds of objections that Palmer raises–however, is that the results from twin studies are robust.

Ok, here is a final and telling point.  Palmer argues that “Mutations and environmental factors cause measurable changes to the genome as life progresses.” Now that is true but you can judge how eager Palmer is to discredit twin studies regardless of the science by how he quickly concludes from this something which is truly laughable:

By the time a pair of twins reaches middle age, it’s very difficult to make any assumptions whatsoever about the similarity of their genes.


Comments for this post are closed