Twin studies have played a major role in investigations of genetic influences on intelligence and personality. Generally, they suggest that genetic influences on those traits are quite strong, a conclusion that a lot of people have had trouble swallowing. As far as I can see, that gag reflex was entirely due to dislike of the implications of a strong genetic influence on human behavior, rather than any compelling argument or solid counter-evidence.
There have always been other approaches to investigating these questions, looking at adoption, varied degrees of relatedness, etc. They produced generally similar results. The latest and most powerful version is the Visscherian approach, which uses overall genetic relatedness and is not affected by most of the possible problems with twin studies. Eric Turkheimer put it this way: “Thanks to the Visscher program of research, it should now be impossible to argue that the whole body of quantitative genetic research showing the universal importance of genes for human development was somehow based on a sanguine view of the equal environment assumption in twin studies, putting an end to an entire misguided school of thought among traditional opponents of classical quantitative (and by association behavioral) genetics.”
I don’t think you get very far by talking about how the world ought to work. Although you can sometimes get somewhere by deluding other people , and maybe that’s what some of these people were up to. They were pretty successful: lots of people think that intelligence isn’t heritable. For some reason, people with such views have lower-than-average birth rates, so maybe it is true, for them.
In general, though, I think people like Leon Kamin deceived themselves first. Anyone who grew up in the US and came to the conclusion that Uncle Joe was a great guy wasn’t exactly a fountain of common sense in the first place.