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As Protagoras points out here there are systematic problems with twin studies.
There are problems, but I don't think they are large, I think they are brought up mostly for ideological reasons (Shalizi is not an unbiased source and has a very big axe to grind), and a lot of the problems also cut the other way. For example, measurement error can reduce estimates of heritability a great deal, as we see in twin studies which correct for it and as predicted get higher heritability estimates, like "Not by Twins Alone: Using the Extended Family Design to Investigate Genetic Influence on Political Beliefs", Hatemi et al 2010 (this ... (read more)