"Twin studies are pretty much useless," says Slate's Brian Palmer. The idea of using twins to track heritability is an old one, but critics say the idea is worn out and can lead to potentially ambiguous conclusions about how genes are inherited and how they affect a person compared to the person's environment. Despite numerous indications that conclusions drawn from twin studies are "deeply flawed," researchers continue to do these studies and write papers based on the results, Palmer says. "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)," Palmer adds. "The first is demonstrably and absolutely untrue, while the second has never been proven." Not to say that genetics don't play a role in behavior, but twin studies aren't the right way to measure that influence, he says.
But at the Gene Expression blog, Razib Khan disagrees with Palmer's conclusions. While the idea of it is bold, the reasoning behind the argument is "just a sloppy mishmash," Khan says. There's a lot more to twin studies than behavior genetics. "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," Khan says. "Today genetic relatedness can be pinned down on an individual basis with much greater precision." That's not to say that all the criticisms leveled at twin studies by Palmer weren't valid, but simply calling them useless doesn't suffice, he adds.