It is a terrible thing when science is hijacked by ideology.
That’s what has happened to the science of breastfeeding, where breastfeeding proponents are falling all over themselves to “prove” that breastfeeding increases IQ. The latest paper in the genre is Infant Feeding and Childhood Cognition at Ages 3 and 7 Years; Effects of Breastfeeding Duration and Exclusivity, by Belfort et al., published yesterday in JAMA Pediatrics.
The study was hyped on NBC.com:
Young children who were breastfed as infants scored higher on intelligence tests than formula-fed kids, and the longer and more exclusively they were breastfed, the greater the difference …
The researchers found that 7-year-olds whose moms had done any breastfeeding during the child’s first year – exclusively or in combination with formula – gained a little more than a third of a point in verbal IQ for each month of breastfeeding compared to children who were never breastfed. That means if the mom did any mix of breastfeeding for the entire 12 months, the gain would be 4.2 verbal IQ points
That sounds very impressive, but, in reality, is basically meaningless. Why?
Suppose I told you that Jane has an IQ of 109 and that Mary has an IQ of 113. And suppose I also told you that the standard error of measurement in IQ tests is approximately 3 points, meaning that Jane’s “real” IQ is between 106 and 112, and Mary’s IQ is 110-116.
Would you conclude that Mary is “smarter” than Jane? Would that mean that Mary would necessarily get better grades than Jane in high school or college? Would it mean that Mary would necessarily have a higher SAT score than Jane? Would it mean that Mary was destined to have greater professional success than Jane?
The answers, of course, are no, no, no, and no.
Indeed, if Jane and Mary were sisters, there would be no way that anyone, including their parents, could identify which girl had the higher IQ. Given the standard error of measurement of the test, it is entirely possible that Jane and Mary actually have the same IQ since, theoretically, both could have a real IQ of 110.
In fact, the study also showed that breastfeeding had no impact on scores of two other tests, WRAVMA (Wide Range Assessment of Visual Motor Abilities) and WRAML (Wide Range Assessment of Memory and Learning), tests of actual skills.
In other words, the study showed that the gains in IQ were trivial and did not translate to a measurable difference in performance on tests of learning and memory.
That doesn’t even take into account the limitations of the study itself (fully 1/3 of children were lost to followup and simply ignored) or the effect of publication bias, which Wikipedia defines as:
a bias towards reporting significant results, despite the fact that studies with significant results do not appear to be superior to studies with a null result with respect to quality of design. It has been found that statistically significant results are three times more likely to be published than papers affirming a null result.
In other words, a study that shows that breastfeeding increases IQ is 3 times more likely to be published than a study that shows breastfeeding provides no benefit in IQ even though the second study may be of equal or higher quality.
I expressed my skepticism of this study to the Wall Street Journal:
Dr. Amy Tuteur, an obstetrician who writes a blog called skepticalob.com, is unconvinced by a four-point increase in IQ, saying the bump needs to be bigger to prove that it isn’t just random variation. “Intelligence is multifactoral and the idea that any one thing can make a big difference right away makes me skeptical,” she said. “American IQ has been increasing steadily, it rose when breast-feeding rates were going down and it rose when breast-feeding rates were going up.”
What the real take home message of this study?
It’s exactly the same as the take home message of all the other over-hyped studies that purport to show that breastfeeding increases IQ:
The impact of breastfeeding on IQ is trivial and has no measurable effect on learning, memory or achievement.
In other words, this study, which will be used as a cudgel by lactivists to shame women who don’t breastfeed, actually provides tremendous reassurance. There is NO need to feel guilty about not breastfeeding, since there is no clinically apparent benefit in achievement.
