Breastfeeding and intelligence: the problem with the newest research

Do I believe the end result?  Probably.  But I don't believe that this paper comes close to proving its case.  Fox News has this discussion of a recent study in JAMA Pediatrics.
. . . Belfort and her colleagues gathered data from 1,312 mothers and children in the United States, tracking everything from the mother’s frequency of breastfeeding to other factors including the mother’s intelligence, the mother and father’s education levels, measures of the home environment, the mother’s employment and the type of childcare the baby received.
While the link between breastfeeding and cognition had been previously explored, many earlier studies did not control for these additional factors.
“Many previous studies have been criticized because any link you might observe between breastfeeding and childhood intelligence could be explained by those other factors,” Belfort told FoxNews.com.
Belfort then performed a series of tests measuring the children’s cognitive development after infancy.  At age 3, the children underwent the higher Peabody Picture Vocabulary Test, a measure of receptive language, or how well a child understands language.
“A child’s receptive language is highly correlated with general intelligence as measured by more typical IQ tests,” Belfort said. . . . 
Unfortunately, the discussion at Fox News is really is too uncritical.  Of course, the bigger blame should be placed on JAMA Pediatrics   As with almost all medical studies, they ignore the issue of endogeniety when they can't do randomized experiments.  If I were do the experiment, I would control for family specific effects (that is a dummy variable for each family to pick up the average intelligence of children born into that family) and, after accounting for birth order, spacing between children, and the other controls included in the study, see if intelligence of children within the family varied with the amount of breastfeeding.  The problem with the current purely cross-sectional study is that the measurement error in how education and other factors are measured is large compared the differences they claim to find in intelligence.

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