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Thursday, March 25, 2010

SES and IQ

It is sometimes claimed that IQ is just a proxy for SES (Socioeconomic Status): high IQ kids are merely the beneficiaries of better home environments, and correlations between IQ and life outcomes are merely a proxy for correlations between childhood environment and outcomes. Of course, this claim does not address the significant variations in IQ within families. Does IQ have predictive power once SES is controlled for? The answer is obvious from anecdotal experience: we all know siblings who, by definition, shared the same family SES, but with different IQs and life outcomes.

Vanderbilt psychologist David Lubinski sent me a recent paper addressing this issue -- Cognitive Epidemiology: With emphasis on untangling cognitive ability and socioeconomic status, Intelligence 37 (2009) 625-633. Among the figures is the following one, based on NLSY longitudinal data (the analysis was originally done by Charles Murray). It compares life outcomes of paired siblings (one of each pair required to be in the "normal" reference range of IQ), thereby controlling for SES. As you can see, IQ has a strong impact on education, earnings and social status even after family SES is controlled for. (Click for larger version of figure.)

Similar analysis shows that the correlation between SAT scores and college grades only decreases slightly when SES is controlled for. See also related post on non-shared environmental effects.

Below is a useful SES-SAT (or SES-IQ) syllogism, well supported by the studies at the two links above.

SES does not cause SAT (weakly at most). ***
SES does not predict college success, SAT does.

*** Someone recently asked me about the non-genetic or environmental variation in adult IQ. If we use a heritability range of .7-.9 (as deduced in twin studies; applicable for IQ in late adulthood and over a large range of childhood environments), then the resulting distribution of IQ differences for monozygotic twins would have SD = 5-8 points. That is, clones raised in different families would typically have IQ differences of around 5-8 points. Note, though, that very little of this difference is accounted for by SES -- most of it is due to non-shared environmental effects. For a lower heritability value such as .5 (perhaps more appropriate for adolescents), the expected variation between clones would be about 10 points.

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