Breastfeeding and IQ
There is an elegantly done study just out in Lancet Which shows that, among Brazilians, children who were breastfed for some time grew up to have slightly higher IQs. The study title is "Association between breastfeeding and intelligence, educational attainment, and income at 30 years of age: a prospective birth cohort study from Brazil".
I really hate to demolish such extensive and careful work but the study has a fatal flaw: Maternal IQ was not measured. And, surprising though it may seem IQ is hugely important to breastfeeding. High IQ mothers breastfeed a lot more. And IQ is of course mainly genetically transmitted. So all that the study really shows is that high IQ mothers have high IQ children, which is no news at all.
In the statistics section of the paper I was delighted to come across a word that I had forgotten I knew: "Heteroscedasticity". I must use it sometime.