Previous studies have also linked breast-feeding to higher IQ, but they generally haven't ruled out the fact that breast-fed kids are also more likely to come from wealthier and better-educated families than formula-fed babies. What has looked like a correlation between nursing and smarts might really have been explained by other more predictable factors. Caspi and Moffitt, however, controlled for the confounding factors of social class and maternal IQ. And they still found a pattern of higher IQs in the breast-fed babies with the C alleles. Nor did the advantage seem related to the mothers' genotypes.
Caspi and Moffitt point out that in the earlier annals of human history, when everyone was breast-fed, "genetic variation in FADS2 could have influenced individual differences in intelligence." In this sense, they say, "It is reasonable to ask whether FADS2 is a 'gene for' IQ." Maybe that helps explain why 90 percent of the population has the C allele—if it promotes intelligence, then it should win out in the process of natural selection. From a pro-breast-feeding point of view, this is all kind of lovely. Some babies have a gene that boosts intelligence, but only when it's activated by the delivery of mother's milk, and so, over time, more babies have the gene. Nature and nurture working in tandem, and to a good end.
OK, enough with the cave folk. What about us? Breast-feeding helps make most kids smarter, if the results of this study bear out in future research. In a birth order study that came out earlier this year, much was made of a three-point IQ edge for firstborns, so the nearly seven-point jump associated with breast-feeding and the C allele looks substantial. But the nursing doesn't help all kids—and so it's actually one more source of heightened inequity. Not just in the sense that kids who aren't breast-fed don't get the boost, a "shame-on-mothers" argument we're used to. No, now we have to add on a genetically based inequity. Of course, life is full of those, but they're still discomfiting, and perhaps all the more so in the presence of this sort of G × E interaction. There are two ways to lose out, genetic and experience-based. And to win, you need both to go your way. (We're talking about averages over populations, not one-to-one correlations, of course, but you can still see the point.)
Then there's the question of genetic testing. You could test a child for the C allele, and if she has it, feel even better about nursing her. You'd also feel worse if you can't. And if your baby is in the 10 percent without the lucky gene variant? You can still nurse, of course, but your rationale would be different—and wouldn't you feel ripped off by nature's grand lottery, and a little bitter about it, at least on a sore-nipple day?
This is the kind of information we may not really want to have. The sinisterish ramifications are even starker in other research that Caspi and Moffitt have done, in which they've found a G × E relationship for a gene that's linked to depression in connection with an experience of serious trauma, like child abuse. Do we want to know who has this predisposition? (A test already exists.) If you were thinking about adopting a child who might have had some hard experiences, wouldn't you choose the child with the protective variant of the gene over the one without it? And what does that mean for the genetically unblessed kid? If you ran an adoption agency, would you give kids the test? Questions for the near future, or, really, for now.