Monday, November 12, 2007

Breast-feeding and IQ   posted by p-ter @ 11/12/2007 09:22:00 PM

In an interesting story on the relationship between teen delinquency and sex (long story short: people who concluded early sex caused delinquency unsurprisingly failed to control for genetics and were led astray) I saw this little bit:
A recent study by Scottish researchers asked whether the higher IQs seen in breast-fed children are the result of the breast milk they got or some other factor. By comparing the IQs of sibling pairs in which one was breast-fed and the other not, it found that breast milk is irrelevant to IQ and that the mother's IQ explains both the decision to breast-feed and her children's IQ.
Now, this is interesting in light of the recent study claiming to find a gene-environment interaction between breast-feeding and a particular gene. The source for the claim that breast-feeding has no effect on IQ is here. I went back and looked at the recent paper's attempts at controlling for maternal IQ. Statstically, this is not a difficult thing to do-- a linear regression of child IQ on maternal IQ, breast feeding status and genotype can easily be compared with a model that includes a breast feeding staus X genotype interaction.

The authors don't do this standard analysis, however--they only include a cryptic note explaining that there is no significant "interaction" between the SNP in question and maternal IQ. It's not the interaction term that's interesting, of course; it's whether the marginal effect of maternal IQ removes their already tenuous claims of an interaction between breast feeding and genotype. One gets the distinct feeling that some unfavorable results are being swept under the rug. Combine this, plus the study above, then add your prior probability that by genotyping two (2!) SNPs in the entire genome you'll find a real gene-environment interaction, and, well, it's not a stretch to say the authors haven't quite demonstrated what they think they have.

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