The key issue is that there could be a link. Because of this possibility, you need to control for genes to reach any strong conclusions. For example, you could

The key issue is that there could be a link. Because of this possibility, you need to control for genes to reach any strong conclusions. For example, you could check to see if the verbal ability of adoptive children more closely resembles the ability of their (adopted) siblings or of their biological parents. Or you could compare twins reared apart to see if their abilities more closely track each other's or those of their adoptive parents. The Slate article you linked doesn't deal with nature vs. nurture at all. It simply assumes that nature doesn't matter, approvingly citing the views of one researcher who makes the astonishing claim that "Children aren’t born smart. They’re made smart by their parents talking to them." This view is definitively disproven both by studies of adoptive children and by studies of twins reared apart (not to mention common sense—cognitive ability must be at least partly heritable, else there would be no way for ever smarter animals to have evolved). For more details, I recommend The Blank Slate by Stephen Pinker.