In this study of the 50 states, parasite prevalence (measured as the infectious disease rate) is strongly associated with lower mean IQ for all races combined (it explains almost half the variation in mean IQ). The authors interpret this to support the view that a distressed immune system during periods when the brain is developing results in a tradeoff that hurts IQ.
The problem is that, according to their data, the correlation between the infectious disease rate and percent black in a state is .90--the two measures are highly collinear. Childhood infectious disease rates are much higher among blacks, but in the U.S. the number of cases of cholera, measles, meningitis, pertussis, rubella, tetanus, or TB even among blacks is low. If parasite prevalence was a major cause of low IQ, the distribution should not be a bell-shaped curve but a small percentage of low-IQ individuals and a high percentage of people with normal IQs. Parasite prevalence could contribute something to low IQ, but I don't see how it could explain almost half of the U.S. variation in cognitive ability. The researchers should have examined race-specific data.
Parasite prevalence that the authors make such a big deal about looks to be simply a proxy for race.
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Hey, haven't you heard? "Correlation doesn't imply causation." Applying observations to shape your beliefs is, therefore, invalid -- even if your observations are of controlled experiments. I mean changing a single variable between two groups merely creates 2 with 0 degrees of freedom no less!
ReplyDeletePS: Sorry, after my rather bitter history of trying to educate folks by presenting exhaustive ranges of ecological correlations and their relative coefficients of determination, I'm rather cynical.
PPS: I'd really like to see someone in the social sciences use Kolmogorov complexity to rank-order models.
It is ridiculous that they did not study whether race might explain their findings better even though the NAEP scores they use as an IQ proxy are readily available for each race separately.
ReplyDelete"If parasite prevalence was a major cause of low IQ, the distribution should not be a bell-shaped curve but a small percentage of low-IQ individuals and a high percentage of people with normal IQs"
ReplyDeleteThey were concerned with between
state variance not between individual
variance.
Their theory is at the individual-level. They test it at the macrolevel.
ReplyDelete"Their theory is at the individual-level. They test it at the macrolevel."
ReplyDeleteYes, it's on the individual level between regional populations (countries and states), bur not between individuals in the same region. Parasite load can suppress the IQ in the Southern US as compared to the North without affecting the distribution of IQ (or heritability) in both regions.