In a decade or two, people may look back on us as having lived in the dark ages, blind to the obvious, and bound up with delusions, angels and devils. Our behavioural science research designs may look hopelessly simple, and wide open to confounding effects. Full genomic analyses may go a long way to resolving some of those confounders, but good research designs will always be required.
Here is a simple question which dates back to 1929: does breastfeeding a child, as opposed to giving them formula milk, boost their intelligence? Set aside for a moment the potential benefits of bonding with the child, protecting them from illnesses, and thoroughly irritating passers-by who hate the sight of humanity, and just concentrate on that question.
Walfisch, Sermer, Cressman and Koren have done just that in a BMJ paper: “Breast milk and cognitive development—the role of confounders: a systematic review” Their abstract immediately hits the nail on the head: “The association between breastfeeding and child cognitive development is conflicted by studies reporting positive and null effects. This relationship may be confounded by factors associated with breastfeeding, specifically maternal socioeconomic class and IQ.”
They do a systematic review of the literature, and found 84 studies met their inclusion criteria (34 rated as high quality, 26 moderate and 24 low quality).
They explain: “Well-established confounders in breastfeeding research include demographic and IQ differences between mothers who breastfeed and those who choose not to. Parents who score high on a range of cognitive abilities have children with above average IQ scores. In parallel, advantage in mother's IQ more than doubles the odds of breastfeeding. Thus, some of the published data demonstrates the disappearance of the breastfeeding effect on child's cognition after correction for maternal IQ.”
“Given that more tight control of confounders resulted in greater likelihood of disappearance of breastfeeding effect, it can be argued that the remaining positive effect reflects residual uncontrolled bias, as shown by Der et al in their large study. In that study, before adjustment, breastfeeding was associated with an increase of around 4 points in mental ability. Post hoc analysis revealed that adjustment for maternal intelligence accounted for most of this effect—where full adjustment for a range of relevant confounders yielded a small (0.52) and non-significant effect size (95% CI −0.19 to 1.23).”
They conclude: “Much of the reported effect of breastfeeding on child neurodevelopment is due to confounding. It is unlikely that additional work will change the current synthesis. Future studies should attempt to rigorously control for all important confounders. Alternatively, study designs using sibling cohorts discordant for breastfeeding may yield more robust conclusions.”
My conclusion: Breast feeding is probably a good thing, but don’t do it with the sole purpose of boosting the intelligence of your child. In all probability, the only way to boost the IQ of your child is to make a careful choice of mate 9 months before. So there’s a headline: “Mate selection more important for your child than breast feeding”. Worth a tweet at least.
BMJ Open 2013;3:e003259 doi:10.1136/bmjopen-2013-003259