Tuesday 27 August 2013

Breast feeding, intelligence, and confounded researchers


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


  1. "In all probability, the only way to boost the IQ of your child is to make a careful choice of mate 9 months before." Yeah, and bonk only on Tuesdays in the September.

  2. amen that's the biggest semi-controllable factor. S. Sailer pointed out a while back if you have, say, 8 children with a woman with an IQ of 120, vs maybe 2 kids with a woman with an IQ of 130, that even with regression to the mean, having more kids thru a slightly lower IQ woman raises your odds of having a kid or 2 over 130 (vs. having fewer kids with a higher IQ woman. he did the math(s:)
    it is odd to me that the few solid things we have in psych (tests, higher heritability the higher the g of the measure, lack of test bias) are played down by psych - & are not encouraged to be known by the general public - but instead psych promotes feel-good hoo-hah like "positive psychology!"

  3. Women who breastfeed are those with the time and opportunity to do so.

    A poor family is more likely to both parents working (full-time if not multiple jobs) and the mother in particular constantly busy even when not at work. Poor mothers have no choice but to rely more on formula. Also, because they are busier, they have less time to read to their children and do others activities that have been proven to increase cognitive ability. Besides issues of formula, poor mothers are more likely to be malnutritioned and so are their children, a major cause of decreased cognitive development. Living in a stressful environment, lead paint in old houses, and on and on.

    Maybe if researchers simply took into account all the complex confounding factors of socio-economic class most of the IQ differences would disappear. The best way to boost IQ in the population would be to solve the problems of poverty, economic inequality and social mobility. But those are the very problems most of those among the middle and upper classes, including scientists, don't want to face.

  4. Thanks for your comments. In fact, until relatively recently, it was the poor who breast fed their children as a matter of course. Still happens in the poor parts of South America, and among poorest groups there. Poverty seems very likely to reduce cognitive development, but the pattern across the world does not match very well. The very poor far East did better than the much richer Gulf region, where two generations of oil wealth and heavy investment in education have not led commensurate scholastic achievements. Other posts on my blog (with Rindermann, and also Meisenberg) look at recent publications on these issues. Poverty is part of the picture, but other factors seem to be greater.

  5. Just postulating...

    Since breastfeeding has been around for millions of years, can we start from the premise that breastfeeding produces normal human beings -- ones that fall in the normal range of intelligence and then from there as breastfeeding as our baseline, look at formula's impact on intelligence?

    I have never understood why we look at it from the other angle, when formula is the newer factor in how we feed our children...

  6. riam you are right that breastfeeding has been the baseline condition for humans, so it has worked well. The problem comes with the contemporary fact that many women in the wealthy world now find formula feed to be convenient, and one cannot conduct a proper controlled trial. Confounded conditions lead to confounded conclusions.

  7. I have never understood why we look at it from the other angle, when formula is the newer factor in how we feed our children...
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