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In 2015, there were two conflicting studies about this subject. CNN reported in March, 2015:

"Breast is best" -- you could call it a mantra of sorts that sums up much of today's research on breastfeeding.

Not only does breastfeeding have clear short-term benefits, such as protection from infectious diseases and a reduction in mortality, it's also been shown to be associated with an increase in intelligence.

Prior studies have shown an increase of up to 7.5 IQ points in elementary age children who were breastfed, as well as an increase in verbal, performance and comprehensive IQ in adults.

The latest addition to this perspective is a long-term study of infants born in Pelotas, Brazil, in 1982. Published in Lancet, the study interviewed 5,914 new mothers about their plans for breastfeeding and then followed up to see how they did.

...

What makes this study unique is that it followed the subjects all the way to age 30.

"We were able to follow about 68% of the participants, which is a very good follow-up rate," said Lessa Horta. "We observed that breastfeeding was positively associated with performance and intelligence at 30 years old, as well as with education, school achievement and higher monthly incomes."

In fact, Lessa Horta said the subjects who had been breastfed for 12 months or longer had a higher IQ (about 3.7 points), more years of education and earned roughly 20% more than the average income level.

"It's suggesting that the positive effect of breastfeeding on IQ leads to a higher income," he said. "This is our main finding at this moment."

But a few months later, another study came out disputing that breastfeeding has no affect in IQ. Telegraph reports:

Breastfeeding does not improve a child's intelligence, despite the widespread belief that "breast is best" for IQ, according to a new study.

Scientists who conducted research on 11,000 British children found no reliable association between breastfeeding and higher IQ at age two.

Nor was breastfeeding related to improvements in intelligence as children grew up.

These studies contradict each other. So, my question is, is breastfeeding correlated with an increased IQ?

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    I'll note that a major problem here is that there are many aspects of breast feeding which might (or might not) affect IQ. First, the proven health benefits could result in higher IQ due to a lowered risk of brain-damaging infections. Then the "together time" between mother and child may have a significant effect. There might be some physical effect due to suckling (that isn't gotten from a bottle) that affects IQ. And, finally, something in the breast milk could affect IQ. (And all of theses possible effects may be mediated by socio-economic factors.) – Daniel R Hicks May 28 '18 at 2:14
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    Or just the mother caring enough and having enough time to go through the effort of breast-feeding might factor in to it. – Erik May 28 '18 at 14:56
  • @Erik - That's the socio-economic thing. – Daniel R Hicks May 29 '18 at 21:16
  • The second study corrected for soceioeconomic facts by looking at twins in the same family. The one cited by CNN did not. – swbarnes2 Jun 4 '18 at 23:43
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There was a prominent randomized control trial in Belarus, called PROBIT.

They did not directly control for breastfeeding, but did control for breastfeeding intervention. Study participants were assigned randomly to two groups, one received an intervention that promoted breast feeding, the other did not. The study was quite large with 17,046 mother child pair participants. They did find a significant reported increase in breastfeeding in the intervention group. They also found a small but consistently higher verbal and overall IQ scores in the intervention group at two ages. These findings were statistically significant.

At 6.5 years, children randomised to the breastfeeding promotion intervention in PROBIT had a 7.5-point advantage in verbal IQ (95% CI 0.8 to 14.3), a 2.9-point higher performance IQ (–3.3 to 9.1), and a 5.9-point higher full-scale IQ (–1.0 to 12.8) than those in the control arm. Smaller but consistently positive differences of 2–3 IQ points were seen in an audit of 190 children and blinded teacher rated academic performance.

The study authors report that one possible confounding variable is whether mothers who participated in the intervention had other behaviors that were modified by such an intervention.

As far as it goes, this is likely of the largest RCT ever performed on breastfeeding intervention.

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    Is there a reason you decided not to cite the most recent PROBIT study? ncbi.nlm.nih.gov/pubmed/29677187 "We observed no benefit of a breastfeeding promotion intervention on overall neurocognitive function. The only beneficial effect was on verbal function at age 16. The higher verbal ability is consistent with results observed at early school age; however, the effect size was substantially smaller in adolescence." – swbarnes2 Jun 4 '18 at 23:40

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