Looks (though I've barely skimmed it) like good evidence that twin studies say less than one might naively think. Doesn't say anything about Caplan. Care to say a thing or two about what Caplan thinks twin studies say and how it differs from what analysis like that reveals that they say?

(Perhaps I'm just unduly lazy; I was hoping to find an easier way of assessing your claim versus Caplan's than by procuring a copy of Caplan's book, reading it carefully, reading a technical paper on twin studies, examining the particular studies on which Caplan's claims depend, and comparing his use of them with the analysis in the aforementioned technical paper. Of course that's the only way if I want to be really sure, but ... well, I'm lazy and was hoping there might be a shortcut :-).)