Does Breastfeeding Increase IQ & Income? cover

Does Breastfeeding Increase IQ & Income?

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The media loved the idea that breastfeeding improves intelligence. And, even better, that a breastfed baby earns more in adulthood. Who wouldn’t? There have been other studies saying breastfeeding raises IQ (and some that say not). The New England Journal of Medicine‘s JWatch has made its past comments on these studies available free. But this new study, from Brazil and published in The Lancet, attracted so much attention because it went on for more than 30 years, which made it seem particularly persuasive.





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Does Breastfeeding Increase IQ & Income?

30-Year Study

The media loved the idea that breastfeeding improves intelligence. And, even better, that a breastfed baby earns more in adulthood. Who wouldn’t?

There have been other studies saying breastfeeding raises IQ (and some that say not). The New England Journal of Medicine‘s JWatch has made its past comments on these studies available free. But this new study, from Brazil and published in The Lancet, attracted so much attention because it went on for more than 30 years, which made it seem particularly persuasive.

Confounding Variables

Confounding Variables

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Other Opinions

Even venues where one might expect critical consideration reported with little analysis, The New York Times health blog, for instance, Vox’s Julia Belluz, an unusually skeptical eye, was pretty accepting of the Brazil study.

There is much to praise in this ambitious long-term study. Spare a thought just for the amount of work involved over decades. At the Guardian’s Brain Flapping blog, Dean Burnett’s critical take begins with praise: “It’s a well-designed study which tries to take into account as many variable and confounding factors that could distort the results as possible.”

Personal Filter

Burnett’s critique leans heavily on the fact that IQ was measured with the always-controversial Wechsler Adult Intelligence Scale.

He also wonders how this widely-praised study will increase the already heavy pressure on new mothers to breastfeed. This concern he bolstered with personal experience: his wife’s breastfeeding instructor, a health care worker, vilified women who considered formula feeding.

Questions Surrounding The Study

Adrienne LaFrance based her Atlantic criticism largely on details of the study–especially that so many of the study subjects enrolled in 1982 (5914 of them) were, as population researchers say, “lost to follow-up.”

After 30 years, only 3493 were still available. That’s perhaps understandable in such a long-term study, but it does raise questions about the data’s validity. She points out other difficulties too. Accounts of breastfeeding practices were collected years after the births, and were based on self-reports by the mothers. Self-reports are intrinsically subject to question. “And even if breastfed babies really do grow up to have higher IQs and make more money, how do we know it’s because they were breastfed?” LaFrance asks.

A Mother’s Disposition

A Mother’s Disposition

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Making Comparisons

Although social class differences in breastfeeding practices seem to be less important in Brazil than in many other countries, it still is the case that nursing mothers tend to be better off and more educated than mothers who don’t (or can’t) breastfeed.

There were other reported adult differences between those breastfed as babies and those who were not. The researchers compared babies who were breastfed for a month or less with those who were breastfed for a year or more.

Statistical Significance

The differences were statistically significant, but not enormous. The income difference amounted to a little over $100 per month at today’s exchange rate, and as with IQ can perhaps be explained by other advantages in the breastfed.

Breastfed children stayed in school longer, but less than a year longer. Also, IQ differences amounted to less than 4 points on average. LaFrance concludes, “And though breastfeeding may play a role in how a kid turns out, 30 years of parenting surely has more to do with what kind of adult a baby becomes.”

This article originally appeared on PLOS.com

(CC BY 4.0)