Could you help me find where they controlled for family income in this particular study? The thing most correlated with IQ test score is your parents wealth (and childhood pleasure reading.) If you then say that IQ score is then correlated with income, it's just saying that rich parents have rich children, and laundering that fact through theories about the existence and detection of general intelligence.

You'd want to look for radical changes in income from parents to children, and look for reflections of that in IQ scores.