Breast milk, genes, and IQ

Few commercially produced health products offer as many benefits. According to popular—and medical—opinion, human breast milk is more or less a superdrug. Scientists agree that breast fed infants end up better nourished than formula-fed counterparts, are less likely to become obese, suffer less diarrhea, and have stronger immune systems. It is no surprise that groups such as the World Health Organization refer to breast milk as the “perfect food”.

As well as the physical health benefits, there is little doubt but that breast feeding strikes a chord with parents who are generally concerned about the impact of technology on planet Earth. Rather than invest money in market-based pharmaceutical or agriscience products, parents can use breast milk to provide natural and ethical sustenance to their children. Breast milk springs directly from nature, untouched by human hands. It advertises the value of the (naked) human body and of femininity, and benefits poor as well as rich families. In other words, not only is it nutritionally awesome, but fashionably radical and politically correct as well.

However, critics often claim that breast feeding advocacy is more ideologically charged than scientifically based, and that proponents of breast milk are occasionally guilty of over-reach. One such claim is the suggestion that breast feeding can serve to boost children’s intelligence.

The idea that breast fed children have higher IQ scores is nearly as old as the practice of measuring intelligence using IQ tests. In 1929, a study published in the Journal of the American Medical Association found that breast fed elementary school children were “definitely superior” to other children in mental development.  Since that time, a number of subsequent studies have claimed to have corroborated this effect. So much so that the assertion is regularly presented by mainstream advocacy organizations. In its promotional material for World Breastfeeding Week, the World Alliance for Breastfeeding Action summarizes the disadvantages of formula feeding as “poorer health, higher mortality rates, and lower IQ scores.” ProMoM, a New York-based lobby group, lists the association between formula feeding and “lower IQ” as #7 in its list of “101 reasons to breast feed your child”. A similar position is proffered by the Breastfeeding Advocacy Network, who describe their campaign as “lactivism”.

However, the science behind the claim is quite loose. Most of the available research is of two kinds.  In the first, data are collected in maternity wards from preterm babies who are given mothers’ milk by tube. When these children are followed up, it appears in a number of studies that they have higher IQ scores. The researchers usually argue that the fact the babies were fed by tube proves that it is the milk, and not the comfort or intimacy of mother-child interactions during breast feeding, which boosts IQ. However, other researchers point out that these children may receive greater general care and attention during later infancy precisely because of the medically perilous nature of their births. They also argue that there are too few studies of this kind, and that you can’t readily generalize such findings from preterm infants to other children.

In the second type of research, which is far more common, data are collected from the general population to see if children who were breast fed as infants have higher IQs in later life. Critics raise many problems with this approach.  For one thing, the data on whether children were breast fed as babies is historical, second-hand, and possibly unreliable. However, the main methodological problem is that it will not be obvious why exactly a particular child will be breast fed and another one not.

As researchers cannot use random allocation to determine who gets the breast milk, they can’t be sure that the differences in IQ are actually caused by breast feeding. They could be caused by whatever it is that led to the mothers to breast feed in the first place. Researchers call these underlying causes “confounders”. One possible confounder is family income. It is well established that higher socioeconomic status moms are more likely than poorer ones to choose to breast feed. If that is the case, then breast fed children are more likely to come from wealthier homes, to have greater access to educational toys, and to be sent to schools which emphasize cognitive development from an early age. If these children turn out to have better test scores, then breast milk seems one of the least likely causes.

When confounders are controlled for, the link between breast milk and IQ disappears. In fact, it appears that it is the mother’s own IQ that is the most important factor. In one study of 5,400 mothers and babies, controlling for maternal IQ reduced the link by 75% compared to previous studies, while controlling for all suspected confounders wiped it out altogether. The research team — led by statistician Geoff Der of the MRC Social and Public Health Sciences Unit in Glasgow, Scotland — also checked the existing literature on breast feeding and IQ, which covered over 8,000 children, and found that relatively few prior studies had included maternal intelligence in their analyses. Those that did tended to find no effect for breast feeding unless confounders were excluded. In other words, if a study reports an association between breast feeding and intelligence, you can be pretty sure that it failed to include maternal and demographic factors that Der and his colleagues have now demonstrated to be crucial.

That said, breast feeding remains important for the physical health of infants, and crucial to the well being of children in non-industrialized nations. Breast feeding helps children to avoid contaminated water and provides a well tailored mix of fresh nutrients. In fact, there appears to be ample justification for breast feeding without having to make claims about its impact on children’s intelligence.

In fact, by insisting on a role for breast feeding in child intelligence, campaigners only succeed in reinforcing the social gap that exists between rich and poor. The Der team found that low maternal education, economic poverty, and poor quality home environments were far more detrimental to children’s IQ scores than formula milk. By focusing on breast feeding, and by dismissing the findings of researchers like Der and colleagues, campaigners may be drawing attention away from important humanitarian and social justice concerns.

And unlike breast feeding mothers, who are disproportionately drawn from the upper socioeconomic levels of society, poor and poorly educated women have few high-profile advocacy organizations to speak out on their behalf.

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Also on this topic on The Science Bit:
On your breast behaviour
A fishy tale: Omega-3, Alzheimer’s, and IQ

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