I think you're essentially right, but just out of curiosity, what are the 'twin-adoption studies' he refers to? As far as I know, there are really very few twins where one twin has grown up entirely separate from the other.
Yes, separated twins are rare. But twins and adoption by themselves are common and can be studied separately. Either one alone can separate nature from nurture. And they both show the same result: 50% genetics, 0% parenting.
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