A few weeks ago I reported some research that seemed to show a relationship between a gain in IQ due to breastfeeding and a particular genetic variant. Looks like I spoke too soon. p-ter has the goods:
The fact that they have a measure of maternal IQ but don't directly include it in the published multiple regression suggests that they tried it, but didn't like the results. They didn't include parental phenotype in any of their previous studies, but there, at least, there was some functional evidence linking the polymorphism and the phenotype. Here, there's nothing. Considering the fact mentioned in a previous post that other researchers find absolutely no evidence for link between IQ and breastfeeding (the entire basis for this study), this has to be classified as highly questionable. And regardless of the veracity of any gene-environment interactions here, the huge effects of breastfeeding on IQ shown by the authors are clearly artefacts of the heritability of IQ, and it's unfortunate that they are being propagated.
This is the classic problem that Judith Rich Harris points on in her work, many researchers just don't control for the correlation between parents and offspring genetically.