The studies seem to be confusing causation and correlation. They seem to assuming that there must be an environmental cause for the differences in people's IQs. This may seem like a stupid assumption to make, but a lot of people in the social sciences take this as a given, even though the weight of evidence is strongly against it.

Perhaps I have missed something, but I would expect that, if they did not have ideological blinkers on they would be saying things like:

"We are taking particular care to separate environmental and genetic factors here by including adopted children, [identical] twins reared separately, other children raised by other than biological relatives".

But I see none of this at all here.

You may be interested to read "The Nurture Assumption" for a detailed analysis of the effect of parents on their children. The basic conclusion is that if you do not seriously abuse your children you don't affect them much one way or the other. Steve Pinker's book "The Blank Slate" is also relevant.