We know breastfeeding helps babies fight infections. Now we're learning it may boost IQ.

For a long time, studies suggested breastfeeding could have a positive effect on a baby's future intelligence and earning power. But we didn't know whether these findings were real because the research was mostly short term or carried out in high-income countries, where wealthier, more educated women are more likely to breastfeed than their lower socioeconomic counterparts.

In other words, it wasn't clear whether the correlation between breastfeeding and IQ was because of the breastfeeding itself or something else — like other advantages that could come with a more favorable socioeconomic status.

Now, a big new Lancet study out of Pelotas, Brazil, suggests it really could be the breast milk that matters when it comes to IQ.

Tracking children from birth finds more breastfeeding is linked to a higher IQ

A woman stands next to a cardboard poster (R) as she breastfeeds her child on the subway platform in New York to mark World Breastfeeding Week. (Timothy Clary/AFP/Getty Images)

Following nearly 3,500 newborns from birth until 30 years old, the researchers looked at how the length of breastfeeding correlated with intelligence in adulthood, educational attainment, and income.

They found that the longer a child was breastfed, the better he or she performed on each of these lifestyle and intelligence measures.

For example, they discovered a four-point difference in IQ at age 30 between babies who were breastfed for less than a month and those who were breastfed for more than a year. This is about a quarter of a standard deviation in IQ.

Those who were breastfed for longer also ended up with about an extra year of schooling and earned 341 more Brazilian reals per month by age 30 (the equivalent of about one-third more than the average income of the group that was breastfed for less than one month).

This study separates breastfeeding from socioeconomic status

The reason these findings are worth paying attention to is simple: in Brazil, unlike in America, the proportion of mothers who breastfeed is about the same in every socioeconomic group.

The Lancet study isn't the first to show that breastfeeding has persistent effects on IQ. This JAMA study from Denmark, for example, found a significant positive association between duration of breastfeeding and intelligence. But this research is the first long-term study to demonstrate that same effect in a developing country setting where the behavior cuts across class lines.

As Michael Kramer, a McGill University professor who has studied breastfeeding, said, "We usually worry about confounding factors by seeing mothers of higher socioeconomic status doing the longer breastfeeding, but this isn't the case in Brazil and other developing countries."

What's more, he explained that other shorter term studies from Pelotas and the Philippines have shown the same effect. As well, Kramer ran a randomized trial of 17,000 infants in Belarus, which also found that breastfeeding had a positive impact on cognitive development.

Still, there may be other variables at play here that the researchers haven't accounted for, and since most of the research has been observational — and not experimental — it can only tell us about a link between the two phenomena (breastfeeding and intelligence) and not that one causes the other. But this latest, high-quality study definitely strengthens a correlation that has been turning up in other studies.

Mothers in America breastfeed less than in many other places

In America, race and wealth correlate with breastfeeding duration. (Wall Street Journal)

The World Health Organization recommends that babies exclusively be breastfed (i.e., only consume mother's milk) for the first six months. After that, it suggests about another year and a half of partial breastfeeding along with baby food. So in total, according to the WHO, babies should be breastfed for about two years. The American Academy of Pediatrics recommends breastfeeding for a minimum of 12 months.

In the US, however, the average duration of breastfeeding is just three months, and America routinely ranks lower than other countries when it comes to the proportion of children who were ever breastfed. There are a variety of social and cultural reasons for this, among them: personal preference, a lack of education about breastfeeding and pervasive formula marketing, short maternity leaves, and the fact that many women find breastfeeding while working to be extremely challenging.

Health benefits of breastfeeding

Health organizations have long promoted more breastfeeding as science has revealed a range of health and social benefits.

One of the undisputed health gains: breastfeeding helps protect against a range of illnesses, including ear, throat, and sinus infections. It also protects against pneumonia and diarrhea, which are common killers of infants globally. Human milk contains many hormones, sugars, cells, and antibodies that aren't found in formula, explained Dr. Kramer, and these have been shown to fight gastrointestinal and respiratory infections in babies. "These things in the milk protect from bacteria getting in, killing them on the spot," he added. "It’s like having antibiotics, but it’s not antibiotics."

The evidence is more mixed and controversial in studies on the correlations between breastfeeding and diabetes, heart disease, hypertension, obesity, and asthma, explained Dr. Kramer, who has examined the totality of the evidence for his Cochrane Collaboration reviews on breastfeeding.

The more exclusive and more prolonged the breastfeeding, the betterAs the latest Lancet study shows, breast milk also seems to confer an IQ boost, but why exactly this is the case isn't yet clear. "It might be the physical or emotional contact between mother and baby," said Dr. Kramer. "It could be the fact that it takes longer to breastfeed, and the mother is talking to the baby so there's more opportunity for verbal exchange, and the baby is getting a faster language acquisition. Or maybe it's something in the milk that hasn’t been discovered."

Again, there's also a chance that the association between intelligence and breastfeeding can be explained by other factors. In observational studies, it's impossible for researchers to rule out all potential confounding variables, and the time between breastfeeding and age 30 is long enough for many other potential influences to be introduced.

Still, Dr. Kramer suggested mothers try to breastfeed wherever possible, and that bottled breast milk is superior to formula, since the latter doesn't contain those helpful antibodies. As he put it simply: "The more exclusive and more prolonged the breastfeeding, the better."

Correction: A previous version of this article misstated the standard deviation for IQ.

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