Does breastfeeding increase IQ or do breastfeeding complications decrease it?

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There’s a new paper on breastfeeding and IQ.

Is breast feeding associated with offspring IQ at age 5? Findings from prospective cohort: Lifestyle During Pregnancy Study yielded surprising results:

[pullquote align=”right” cite=”” link=”” color=”” class=”” size=””]We need to know not merely how long women breastfeed, but why they stop.[/pullquote]

In multivariable linear regression analyses adjusted for potential confounders breast feeding was associated with child IQ at 5 years (categorical χ2 test for overall association p=0.03). Compared with children who were breast fed ≤1 month, children breast fed for 2–3, 4–6, 7–9 and 10 or more months had 3.06 (95% CI 0.39 to 5.72), 2.03 (95% CI −0.38 to 4.44), 3.53 (95% CI 1.18 to 5.87) and 3.28 (95% CI 0.88 to 5.67) points higher IQ after adjustment for core confounders, respectively. There was no dose–response relation and further analyses indicated that the main difference in IQ was between breast feeding ≤1 month versus >1 month.

Here are the results of verbal IQ:


And performance IQ:


The authors concluded:

Breastfeeding duration of 1 month or shorter compared with longer periods was associated with approximately three points lower IQ, but there was no evidence of a dose–response relation in this prospective birth cohort, where we were able to adjust for some of the most critical confounders, including maternal intelligence.

That makes little sense on its face. If breastfeeding truly improves IQ, one would expect a dose response relationship with longer periods of breastfeeding leading to higher IQ. Moreover, one would expect little to no impact from very short periods of breastfeeding.

The authors chose to put a positive spin on a paradoxical result:

Our finding of a three point difference in IQ associated with any duration of breast feeding longer than 1 month is in support of current recommendations, and is even a relaxed message to mothers who struggle with exclusive breast feeding.

But that’s not the only conclusion you could draw. The same data could be used to argue that the babies who breastfed for less than a month were harmed by adverse effects of breastfeeding itself. Instead of increasing IQ, breastfeeding had no impact and breastfeeding complications actually decreased IQ. It’s not a possibility that the authors ever considered since nearly all breastfeeding research starts with the unfounded assumptions that breastfeeding must have benefits and couldn’t have harmful effects.

The study itself has some very real strengths but also some serious weaknesses.

It’s chief weakness is that it reflects secondary findings from a study designed to assess the impact of maternal alcohol intake.

The LPDS (Lifestyle During Pregnancy Study) consists of 3478 mother–child dyads selected from the DNBC with oversampling of pregnant women with moderate weekly alcohol intake, alcohol binge drinkers and women with high versus low fish intake, iron intake and duration of breast feeding, respectively.

Secondary findings are often the result of outcome switching, an issue with serious ramifications for the integrity and reproducibility of the research.

As John Ioannidis and colleagues explain:

Outcome switching refers to the possibility of changing the outcomes of interest in the study depending on the observed results. A researcher may include ten variables that could be considered outcomes of the research, and — once the results are known — intentionally or unintentionally select the subset of outcomes that show statistically significant results as the outcomes of interest. The consequence is an increase in the likelihood that reported results are spurious by leveraging chance, while negative evidence gets ignored. This is one of several related research practices that can inflate spurious findings when analysis decisions are made with knowledge of the observed data, such as selection of models, exclusion rules and covariates. Such data-contingent analysis decisions constitute what has become known as P-hacking …

There’s also the possibility that the variable that was originally studied (in this case alcohol intake) is a confounding factor in any study of the secondary variable (in this case IQ). The authors do acknowledge this problem and attempt to adjust for it.

A major strength of the study is that adjustment for critical confounding variables including, most importantly, maternal IQ. Most studies on breastfeeding and child IQ have failed to take maternal IQ into account, which renders their findings highly suspect.

Only 6 out of 1385 women in the study (0.4%) chose not to attempt breastfeeding.

In our study sample, we categorised the shortest duration as ≤1 month, since very few women reported breastfeeding duration shorter than this, reflecting that by far the majority of mothers in Denmark choose to breast feed their children.

The authors understand that those who don’t even try differ from other Danish women in important ways:

Adding to the difficulty of obtaining an exposure group with shorter duration of breast feeding is the fact, that women who from the beginning choose not to breast feed may be different from those who do breast feed; for example, women who rely on medication for various reasons may choose not to breast feed because of concerns that medication in the breastmilk may harm the infant…

So far, so good, but the authors fail to consider that those who stop breastfeeding after less than a month may also differ from other Danish woman in a critical way. Their infants may have suffered medical complications from breastfeeding like dehydration or jaundice from insufficient milk supply. It seems never to have occurred to the authors that breastfeeding can have risks as well as benefits. Since up to 15% of first time mothers may have insufficient breastmilk, particularly in the early days of birth, a substantial proportion of babies will likely suffer serious consequences of any effort to promote exclusive breastfeeding.

The data the authors provide suggest that may indeed be the case:

Women who breast fed for less than 1 month compared with 7–9 and more than 10 months were generally younger, they were more likely to be nulliparous (had not previously given birth), have higher BMI, to have been smokers during pregnancy or to have their children be exposed to tobacco smoke postnatally, and have lower IQ…

Nulliparity and higher BMI are both risk factors for insufficient breastmilk.

More notable was this:


Both maternal IQ and education were linearly associated with breastfeeding duration EXCEPT for duration less than a month. As the chart above shows, Maternal IQ and education were lowest for those who breastfed 2-3 months, whereas IQ and education for those who breastfed for less than a month were equal to the mean for the group. That’s just what you would expect if breastfeeding for less than a month were not a choice but a necessity due to medical factors.

The authors believe they found this:

We found no clear dose response relation of breastfeeding duration with child cognitive development in our data; rather, our results point to a difference in IQ of approximately three points between children who are breast fed for a short period of 1 month or less compared with those who are breast fed longer.

But they may have found the opposite: breastfeeding has no impact on IQ, but breastfeeding complications lead to a decrease in IQ.

There’s one way to find out. We need to know not merely how long women breastfeed, but why they stop. The IQ of children whose mothers chose not to breastfeed for personal — not medical — reasons may be no different than the IQ of children who were breastfed for more than 1 month. That would upend everything we believe we know about the benefits of breastfeeding.

29 Responses to “Does breastfeeding increase IQ or do breastfeeding complications decrease it?”

  1. GeorgiaPeach23
    June 12, 2019 at 8:00 pm #

    Very high quality post, Dr. Amy. I love your data analysis posts. I’m a data scientist, new mom, had a C/S breech baby, and quit BF early following terrible latch problems then failure to thrive. I feel so alienated by the vast majority of pregnancy/birth/baby content out there, and there’s so much scientific misinformation. Dr Amy, Pauline Hull, and Emily Oster have been my Holy Trinity. Thank goodness women like you are out there speaking the truth.

  2. rational thinker
    June 7, 2019 at 6:09 am #

    I have 3 sisters and I am the youngest. We were all breastfed and I am the only one with a high IQ. None of my sisters have an IQ over 95 and one of them could possibly have one in the 70’s. Mine is 130 as of the last time I was tested and my son’s is 120. My moms is probably in the low 90’s or high 80’s and my dad is probably somewhere between 105 and 110. So I really don’t know why mine is so much higher than the rest of my family besides my son. If anyone has a theory on this I would love to hear it. Also when we were growing up I was the favorite so I may have been given more attention maybe cause I was the baby. I have always been a little confused because my IQ is a lot higher than the rest of my family. The only thing I can think of is maybe mom produced more milk by the time she had me #4 so maybe I had more milk for better brain development. Thats the only thing I can come up with.

    • Who?
      June 9, 2019 at 6:47 pm #

      The older I get, and the more little apples I see falling far from the tree, the more I doubt the direct and necessary relationship between family/upbringing and any trait, including IQ.

      I have highly intelligent friends, with highly intelligent partners, whose children are entirely average intellectually. Those particular kids have had every advantage, know how to work hard, they know how to perform and behave, and often go well in their chosen careers more due to application than an abundance of brains. Some unfortunately have come up in very competitive households, feel the deficit, know they can’t ‘match up’ and can’t seem to get past that. Which is a real shame because they have something to offer but don’t know how to find it outside the confines of their environment.

      Sometimes there’s a ‘musical’ kid, or a kid who’s good at maths or loves to read, in families that have never seen that before. Genes are weird, environment is really weird. The neighbour blasting Mozart, or talking about books to the five year old, might have a huge influence on their way through. The passionate teacher really can open doors.

      My two cents? Brains are like any other talent or attribute over the bestowal of which you have no control: not worth praising. In no particular order, application, effort, passion, kindness are what set us apart from each other.

      • Lancelot Gobbo
        June 12, 2019 at 9:00 am #

        Very smart parents are likely to have kids who are less smart than they are, which is an example of regression to the mean.

        As for rational thinker, there are so many variables. Later pregnancies have shorter labours with less risk of anoxia (but more risk of precipitous delivery and IC hemorrhage). Birth order on average means the first child is smarter. Being born later in your family could also mean more resources were available and thus more stimulation for you. Too many things to know which mattered. It may have been a simple genetic lottery win for you. Just be glad you got that win!

        • Who?
          June 13, 2019 at 5:23 pm #

          Really, wow.

          Processing that. It is counter-intuitive.

  3. Emilie Bishop
    June 6, 2019 at 11:21 pm #

    I am so curious to see if your hypothesis is right, Dr. Amy. You’ve alluded to it in other posts and I honestly think there’s something there. If there is, then the WHO has way more to answer for than we already know they do.

  4. MaineJen
    June 6, 2019 at 1:26 pm #

    3 IQ points.



    • JDM
      June 9, 2019 at 12:21 pm #

      3 points on a notoriously vague test. Not impressive even if accurate.

    • overgrownhobbit
      June 13, 2019 at 2:04 pm #

      Stuff like this reminds me of an undergrad paper I wrote about minute concentrations of pharmaceuticals (mostly statins) and their effects on developing copepods. I lovingly cared for those microscopic insects and watched the experimental group lag behind the control group in development by just a few days, but proved themselves just as fertile as the control group. For the few days after I had analyzed my data but before I had met with my project advisor, I felt like hot stuff, like I just cracked the rosetta stone or whatever else nerds fantasize about.

      When I handed him my data, he made a funny face and made some noise to the effect of “huh …. interesting” and then he unceremoniously told me, “congratulations, you got your first round of boring data”. We discussed the outcomes, and he kindly informed me that while I did have statistically relevant data, the data did not suggest these creatures were under any sort of threat given the parameters of my study.

      So yeah, when I see an effect size of three IQ points, I roll my eyes.

  5. UNCDave
    June 6, 2019 at 12:46 pm #

    Not relevant to the discussion, but looking at the date there appear to be very few single moms in Denmark (although I realize that many who are cohabiting were not married, so it depends on your definition of single), but also that nearly 1/3 smoke during pregnancy.

  6. The Bofa on the Sofa
    June 6, 2019 at 11:37 am #

    Here’s what a 3 point difference in IQ means

  7. Ayr
    June 6, 2019 at 7:14 am #

    My husband had two brothers, one of them passed away in July 2016, all three were breastfed, the oldest was fairly smart, my husband (he’s the youngest) is brilliant the man can do calculus in his head, he uses an app on his phone that requires him to solve math problems to turn his alarm off, the middle brother is dumber than a box of rocks, he barely knows how to tie his shoes (there are so many other examples). My brother and I were both formula fed and we aren’t stupid, both in advance placement classes all through school, my parents had my brother’s IQ tested at 9 and he his IQ was just a few points below that of Einstein, so in short I call BS on this study.

  8. rational thinker
    June 6, 2019 at 6:37 am #

    I think they need to do another study but between siblings who were fed different ways and factor in moms IQ.

    • GeorgiaPeach23
      June 12, 2019 at 8:02 pm #

      I am pretty sure they did and the result was that BF didn’t make any statistically significant difference. Birth order might still matter though as first borns tend to get more attention, and that’s difficult to correct for.

  9. Guesty
    June 5, 2019 at 1:53 pm #

    Can you get an accurate IQ on a 5-year-old? I was of the impression that a child’s IQ can easily fluctuate by 5+ points from one day to the next. Did they test each child’s IQ repeatedly and take his/her mean score?

    • Gæst
      June 5, 2019 at 2:55 pm #

      I don’t know how accurate it is, but they test four year olds in NYC all the time for gifted & talented placement, and for support services.

      • Vast
        June 6, 2019 at 1:35 pm #

        Typically it is thought that you’re mostly testing for social class at that age, if for G&T placement. It results in a watered down G&T curriculum that isn’t as suited to the true outliers because when they’re older some of those kids would not have made the cut and it also discriminates against disadvantaged kids who didn’t have a lot of early childhood education. For support services, it’s kind of different because few people would try to cling to that when it’s no longer needed and it’s rather individualized. (But just like some of the kids would no longer qualify for the G&T program, it’s true that some of the “delayed” kids would catch up regardless of any intervention.)

        • Gæst
          June 6, 2019 at 1:51 pm #

          G&T curriculum, HA. From what I hear, the curriculum is the same, they just get more homework. The city does try to make the tests fair, I think, but the problem is who can afford test prep and who can’t (they are currently rolling out free preschool for 3s, so early ed is more inclusive). Kids can test for it up to second grade, and then when jr. high school application begins, everyone starts from the same pool again (that is, no automatic admission to special middle schools for kids who went to g&t primary schools, with a very few exceptions of k-8 gifted programs). I bought one $10 workbook from B&N for my kids and used that plus the city-issued sample questions. My son qualified for and was offered a G&T spot, but in the meantime both twins got an offer at a non-zoned public school that is highly sought after, and since he’s 2e it seemed like a better fit. The biggest problem with testing four-year-olds is that they are not emotionally ready for a high-stakes test. I mean, if my son had not gotten his preferred breakfast that day, he would have refused to comply. And a kid who doesn’t qualify at age four may be only a few years away from a big developmental spurt. I do think there are elementary school kids who are not being served in a gen-ed classroom – the profoundly gifted, in particular. Perhaps with better understanding of giftedness they could be, but in my experience they aren’t now. If they do “good enough” to meet expected benchmarks, then they are set aside to focus on students who are not there yet instead of being given instruction to help them meet their full potential. (Neither of my kids are profoundly gifted, so this isn’t personal.)

          The special ed evals are not really individualized if you go through the DoE, though. You are allowed to submit private assessments, however.

          • Vast
            June 6, 2019 at 2:38 pm #

            In my district (my child isn’t yet in Kindergarten) they do some sort of test mid-year in Kindergarten for… I don’t know, a small pull-out or push-in program for academically advanced kids that lasts through all of elementary school but is separate from the “gifted” program. In first grade, they do a class-wide test for G&T with the CogAT. I think 98th percentile on that and all the achievement tests gets a one day a week pull out program and 99th percentile on that (and at least 98th on all achievement tests?) gets a full-day isolated program. From what I understand, the curriculum is advanced and more in-depth. The requirements are quite strict (multiple tests and they even do a kind of ridiculous “creativity” test with a score cutoff)… maybe so strict that many kids who are highly intelligent will get missed due to having an even slightly off day or being 2e? I’m hoping that turns out okay anyway – it may be that they need to make the requirements so strict because this district is basically packed to the gills with affluent, educated parents who are working in an intellectually demanding field.

          • Gæst
            June 6, 2019 at 3:06 pm #

            Even the public school my kids are in is full of children from affluent, educated parents. I’m certainly educated, but as I’m unemployed at the moment I’m not exactly affluent. But since it’s a non-zoned school, they are also working on a five-year admissions quota to admit more kids from a lower SES. It’s a school that prioritizes diversity, and it’s funny how that leads to lots of affluent white families trying to get in.

  10. TsuDhoNimh
    June 5, 2019 at 12:43 pm #

    Dang … if my mom had only breastfed me, I’d be as smart as my siblings!

    The following things could easily decrease IQ a few points: neonatal jaundice (if it’s bad enough it needs to be treated), dehydration, and calorie deficits in the critical early weeks.

    They need to go back through and check the medical records of the ones whose mothers started out breastfeeding and stopped, and a group whose moms said “No thanks, I’ll be using formula” from day 1.

  11. June 5, 2019 at 12:23 pm #

    Three IQ points? They have to be kidding. That’s not a meaningful difference for adults, let alone 5-year-olds.

    • June 5, 2019 at 1:16 pm #

      My IQ is undoubtedly 10 points lower before breakfast every day, or when the outside temp is above 30C.

      • Tempera
        June 5, 2019 at 2:24 pm #

        The average difference of 3 points on group level could also reflect that there are some individual children who have suffered more from breastfeeding complications (in the form of e.g. learning difficulties) and some who have survived as well as their peers. The difference on a group level is significant and should definitely be researched in more detail from this point of view.

    • Amy Tuteur, MD
      June 5, 2019 at 1:47 pm #

      Here’s what the authors say: “Although negligible when considered at the individual level, seen from a public health perspective a difference of 3 IQ points must be considered substantial, and smaller effects have previously led to quite conservative precautionary recommendations, for example with respect to adverse effects of maternal exposure to environmental toxicants.”

      • PeggySue
        June 5, 2019 at 2:23 pm #

        Hmmmm. The statement that smaller effects have led to quite conservative precautionary recommendations… those effects would be in a negative direction, i.e., suggesting harm, so it seems prudent to be conservative. Though it would be interesting to see whether those recommendations have held up in time.

      • June 5, 2019 at 5:06 pm #

        Oh, okay, the authors recognize the weakness of their argument and are employing special pleading. Got it, thanks.

      • GeorgiaPeach23
        June 12, 2019 at 8:05 pm #

        I daresay the other interpretation might be that we should relax a little about environmental toxicants? OOH, is it additive, can I have whiskey while pregnant if I pinky promise to EBF for 12 months? (Mostly joking on the ridiculous position women are in trying to parse all this conflicting advice and fucky data.)

    • momofone
      June 5, 2019 at 10:17 pm #

      But these are 3 super-magical BREASTFEEDING points!

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