Does nursing really affect your kid's IQ?

Snapshots of life at home.
Nov. 8 2007 5:47 PM

The Breast and the Brightest

Does nursing really affect your kid's IQ?

Illustration by Robert Neubecker. Click image to expand.

In my corner of the world, breast-feeding is not really optional. Among the women with whom I talk about babies and kids, I can't think of anyone I know in my approximate age bracket who didn't try. My friends who quit at three months seemed like rebels. And when I cut off my sons, after more than a year each, I felt a little heartless because I know so many kids who zealously nursed into toddlerhood.

Emily Bazelon Emily Bazelon

Emily Bazelon is a Slate senior editor and the Truman Capote Fellow at Yale Law School. She is the author of Sticks and Stones.

Nursing is cozy and nurturing, not to mention remarkably efficient—never again will I provide for my children's needs so gracefully (OK, that's not the right word for the pumping part of it). But it can also get a little fanatical. Mothers who adopt children are left out. And when breast-feeding doesn't come easily—an unusual but real occurrence—women sometimes go to great and uncomfortable lengths involving all manner of awkward contraptions. They do this because they think that they'll be depriving their babies if they give up as nursing failures. Breast-feeding is supposed to protect against childhood cancer, obesity, allergies, infections, and global warming (I made up only the last one). Some of the claims, like a lower rate of infant respiratory infections, seem to hold up; others, like reduced odds of adult obesity, probably don't.


Now there's new evidence about the gold ring of breast-feeding benefits—extra IQ points. It's a finding with a twist. The researchers report that breast-fed babies get an average IQ advantage of 6.8 points—a nice step up—but only if they carry a certain genetic variant. If you've got the gene and your mother nurses you, she is making you smarter. If you don't have the gene, the nursing is for naught, IQ-wise. What are we to make of this?

Practically speaking, probably nothing. A series of caveats apply. This is only one study, and there are lots of other reasons to breastfeed (or not to). Plus, 90 percent of the population has the genetic variant that conveys the IQ boost, so the odds are in the suckler's favor. But as food for thought, this study has all kinds of goodies. It's a pretty riveting example of a dynamic that scientists call "G × E," for genes times environment—the notion that it's not nature or nurture that exclusively makes people who they are, but nature interacting with nurture. G × E starts cropping up a lot, once you look for it, and the concept makes sense intuitively. Why wouldn't the meshing of our genes and our experiences have its own influence? But G × E doesn't really resolve the hard questions about the haves and have-nots that the nature-nurture debate spawns. It merely reframes them—and opens the door to predict-the-future-of-your-kid information, via genetic testing, that for many parents is equal parts tantalizing and scary.

The new breast-feeding-IQ study was conducted by the lab of Avshalom Caspi and Terrie Moffitt, smart and careful scientists whom I've written about before. Caspi and Moffitt looked at two large groups of kids, one of more than 1,000 from New Zealand and the other of about 2,200 from England. They asked mothers about whether they'd breast-fed via questionnaire, when the kids were 3 and 2 years old (not the best method for data-collection, but at least the kids were still young). In New Zealand, 57 percent of the kids were breast-fed; in England, 48 percent. Then the authors looked at a gene called FADS2. It plays a role in regulating the production of some special fatty acids present in breast milk, which may help spark cognitive development (though there is controversy about this, Slate contributor Sydney Spiesel tells me). There are three variants of this gene. Two of them include an allele (the "C allele") that conveys a significant IQ boost—but only in conjunction with breast-feeding. In other words, if you have the gene but your mother doesn't nurse you, you're a bit dumber than you otherwise would be. If you don't have the lucky allele, you don't get the IQ boost however much mother's milk you imbibe.