But the analysis that we're talking about looked at the IQ of kids relative to their social class. m_b's argument is that part of the reason that people working in menial jobs have, on average, lower IQs is because if they had higher IQs they'd be more likely to have become brain surgeons or bank managers. Which is fair enough, but not really relevant to questions of education - a kid's social class is (afaik) defined by what their parents do, not what they do, so that argument doesn't really apply. Given a correlation, we have to look at their social background as a cause not an effect.