Brian Palmer mounts a forceful argument that twin studies don’t prove what people think they do. Part of his case is to make the point that it’s difficult to draw inferences about the general population from the behavior of identical twins since identical twins end up having unusual life experiences precisely in virtue of being identical twins. He also puts on the table something I hadn’t heard before, namely the idea that identical twins aren’t actually genetically identical:
More than 40 percent of the known copy number variations involve genes that affect human development, and there are strong indications they explain observed differences between monozygotic twins. For example, it’s often the case that one identical twin will end up victimized by a genetically based disease like Parkinson’s while the other does not. This is probably the result of variations in the number of copies of a certain piece of DNA. Copy number variations are also thought to play a role in autism spectrum disorder, schizophrenia, and ADHD, all of which can appear in only one member of a monozygotic twin pair (PDF). If copy number variations can affect discrete and diagnosable disorders, then why shouldn’t they influence far more complex behaviors like your inclination to head to the polls on a Tuesday night in November?
That’s just the beginning of the genetic differences between monozygotic twins. As a result of mutations during development, about one in 10 human brain cells has more or less than the typical two copies of a chromosome. Identical twins also have different mitochondrial DNA, the genetic information stored in the cellar organelle responsible for processing glucose. Research suggests that mitochondrial DNA affects brain size among a host of other neurological traits.
A little bit strangely, though, he ends the piece by saying, “None of this is to say that genetics don’t play some role in our behavior—they absolutely must.” That’s as if to say he thinks that the upshot of these influences is that twin studies are leading people to overrate the role of genetics in producing behavioral outcomes. That doesn’t seem to me to follow. It’s still the case that identical twins are more genetically similar than other kinds of siblings. So if we have a study showing that identical twins are systematically more similar in some respect than non-identical twins, we’re still in possession of evidence about the influence of genetic similarity on behavioral similarity. The point about identical twins possibly being treated differently than non-identical twins is still valid, but the fact that “identical twins” may be less-than-totally identical doesn’t seem to me to do much to undermine the core method.