Breastfeeding does NOT cause higher IQ

There is a 2007 article in PNAS (Proceedings of the National Academy of Sciences), “Moderation of breastfeeding effects on the IQ by genetic variation in fatty acid metabolism,” by Avshalom Caspi et al., claiming to have discovered a genetic variation in the way that breastfeeding effects IQ. However, this is in complete contradiction with a 2006 article published in BMJ (British Medical Journal), “Effect of breast feeding on intelligence in children: prospective study, sibling pairs analysis, and meta-analysis,” by Geoff Der, G. David Batty, and Ian J. Deary, claiming that breastfeeding has no effect on IQ after other confounding factors are accounted for.

The results are completely contradictory. What’s going on here?

Der, Batty and Deary study

The Der, Batty and Deary study uses data from the 1979 National Longitudinal Survey of Youth. It looks at the children of participants in that study, who were tested between the years of 1986 and 2002, so these children were fed formula before supplementation of infant formula with DHA and ARA fatty acids (which are supposed to better duplicate the effect of human breast milk) were available in the United States.

The Der, Batty and Deary study notes the major confounding factor in breast-feeding analyses is that women who breastfeed are usually of higher intelligence and social class than those who don’t breastfeed. In the NLSY79 data, breastfeeding mothers scored about one standard deviation higher on the Armed Forces Qualifying Test (AFQT).

A multiple regression analysis of the NLSY79 data, which includes 5475 children, shows that breast feeding has no statistically significant impact on children’s cognitive assessments. Factors having the strongest correlations with children’s cognitive assessments are the mother’s AFQT score, the HOME cognitive stimulation score (which is probably a good surrogate for the father’s IQ and a host of other socioeconomic factors), and strangely enough, birth order.

The study also did an analysis of 332 sibling pairs where one was breastfed and the other was not. The results show that the formula-fed sibling had an IQ that was 0.63 points higher than the breastfed sibling. The authors didn’t bother to point out that this result was the opposite of the expected pattern. The P value is 0.506, so the authors probably figured this is just a statistically insignificant fluke and not worth mentioning.

There is additional evidence in the study, beyond what I’ve written about here, confirming the conclusion. Based on the Der, Batty and Deary study, I am convinced that the hypothesis that breast feeding causes higher IQ has been proven false. The cause and effect go the other way. Breast feeding predicts having parents with higher IQs and higher socioeconomic status.

Caspi study

The Caspi study claims that breast feeding increases IQ by a whopping 6.8 points (with IQ having a standard deviation of 15) when the child has one or two copies of the C allele of the rs174575 SNP of the FADS2 gene.

Such a large IQ increase attributed to breast feeding is an impossible result given the conclusion of the Der, Batty and Deary study, unless you think that Der, Batty and Deary falsified data.

Looking at Table 1 of the Caspi study, which is apparently what the conclusion of the study is based on, it’s apparent that the Caspi study did not properly take into account confounding factors. The Caspi study does not contain a regression analysis like the Der, Batty and Deary study, nor does it include a sibling analysis. Thus, the Caspi study is useless crap.

I’ve created the following four tables from the data in Table 1 of the Caspi study:

New Zealand cohort, not breastfed
	CC	CG	GG
Sample size	n=183	n=153	n=35
Children's IQ	98.4 (15.2)	95.8 (12.4)	100.3 (11.2)
Maternal IQ	97.1 (15.2)	96.0 (12.9)	100.2 (13.4)

New Zealand cohort, breastfed
	CC	CG	GG
Sample size	n=238	n=201	n=48
Children's IQ	103.2 (13.9)	104.0 (13.4)	98.9 (13.8)
Maternal IQ	102.4 (15.2)	103.6 (14.2)	103.3 (14.9)

British cohort, not breastfed
	CC	CG	GG
Sample size	n=524	n=375	n=61
Children's IQ	97.3 (14.1)	97.2 (13.9)	99.9 (15.3)
Maternal IQ	95.1 (14.9)	98.5 (12.7)	91.8 (14.5)

British cohort, breastfed
	CC	CG	GG
Sample size	n=488	n=337	n=63
Children's IQ	104.0 (15.0)	104.6 (15.3)	100.7 (17.3)
Maternal IQ	105.0 (12.9)	105.2 (13.8)	102.6 (14.4)

“Entries in the table are means and standard deviations. IQ scores were standardized to M = 100 and SD = 15 in each cohort.”

For children with the CC and CG alleles (91.7% of the samples), we observe results completely consistent with the conclusion of the Der, Batty and Deary study, which is that parental IQ and other factors determine the child’s IQ, and not breastfeeding.

Considering only the 91.7% children with CC and CG alleles, I added the numbers together to get the following results:

New Zealand cohort
	Not breastfed	Breastfed
Mother IQ	96.6	102.9
Child IQ	97.2	103.6
Difference	0.6	0.6
British cohort
	Not breastfed	Breastfed
Mother IQ	96.5	105.1
Child IQ	97.3	104.2
Difference	0.7	-0.8

Note that the differences in the tables above may appear off by 0.1 because of rounding error.

The British cohort, which is the larger sample, shows the average breastfed child has an IQ slightly lower than the average breastfeeding mother. The average formula-fed child has an IQ slightly higher than the average formula-feeding mother. These results are as expected. Maternal IQ predicts the child's IQ, plus there is a small amount of regression towards the mean.

In the New Zealand cohort, both breastfed and non-breastfed children have an average IQ that’s 0.6 higher than their mothers’ IQ. I don’t know why there’s no regression towards the mean in this sample. But the sample size is smaller, and the IQ difference between breastfeeding and non-breastfeeding mothers is also smaller.

The weird results occur with the 8.7 % of children with the GG alleles.

In the British cohort, non-breastfed GG children have an average IQ that’s 7.4 points higher than predicted by maternal IQ. Breastfed GG children have an average IQ that’s 3.6 points lower than predicted by maternal IQ. Thus there’s a net 10.9 IQ point benefit from formula feeding over breastfeeding.

In the New Zealand cohort, non-breastfed GG children have an average IQ that’s 0.5 points lower than predicted by maternal IQ, and breastfed GG children have an average IQ that’s 5.0 points lower than predicted by maternal IQ. Thus there’s a net 4.5 IQ point benefit from formula feeding over breastfeeding.

If we average the British and New Zealand results together, taking into account the larger British sample size, there seems to be an 8.3 IQ point benefit to formula feeding when a child has the GG alleles.

This result seems consistent with the finding of the Der, Batty and Deary study where breastfeeding resulted in a 0.63 IQ point boost when sibling pairs were looked at. 8.3% of 8.3 is 0.69. The numbers are pretty close.

Commentary

The Caspi study is a completely crap study, and the people involved with it ought to be ashamed of themselves for promoting junk science, and for scaring mothers into thinking that they are harming their children by not breastfeeding them. And everyone who was involved with publishing the study ought to be ashamed of themselves. There should be accountability. These hacks should be fired.

The mystery of why formula feeding seems to boost IQ when a child has GG alleles obviously needs further investigation. The medical community talks about the alleged benefits of breastfeeding, but it seems that once again the medical community has it all backwards. Based on this data, formula seems like a better idea than breastfeeding.

Why do people want to think that formula is bad for infants? It obviously has to do with the notion we see quite often, especially from the political left, that technology is bad and nature is good. Thus natural breast milk has to be better than formula made in a factory. Furthermore, feeding your baby formula is too easy. Everyone knows that better results require effort. People want to believe that they have more control over their baby’s welfare than they actually have. But it turns out that when it comes to formula vs. breastfeeding, the easy way is actually the best way.

More junk science debunked by Half Sigma.

If you would like an e-copy of both of these studies, send me an email.

Comments

[Pender]

Wow. Thanks for this post. That's quite a conclusion, given how credibly the press took the original story.

[the professor]

People particularly like the soy milk formula alternative. They believe it's healthier and safer (because of lactose-intolerance).

But the reality is different, very different. Studies have shown many dangers of this formula, including the fact that it contains phytoestrogens and isoflavones which forces kids to develop faster. The faster development is not better development. Girls reach the age of menarche much earlier than in the past, and boys get underdeveloped testicles.

Ofc, soy milk formula is a bit extreme, but common. What seems to be the case is the following: the furthest diet the child eats from his natural one, the more problems will have.

It's easy to blog your opinions on this. (medical research is not very scientific, so you have opinions, not research) I want to see what you do when you have a child.

[the professor]

Since you claim you "debunk" junk science, I'm going to point out a few problems in your analysis:

1. Numbers in the study are too small, to be statistically significant.

2. There is no mention how these mothers were chosen to be in the study. They were probably chosen from the same hospital. It's a big sample bias.

3. The difference of 1 point from 105 to 106 is different that the point from 96 to 97. To increase your IQ from 105 to 106 is harder than from 96 to 97. You are comparing these as they are the same.

4. You pointed out a bias in the sample, but you can't account for it and get "the right" results. You'll need to redo the experiment with the same sample.

BOTTOM LINE:
To have conclusive results you need to work with the same sample. (i.e., mothers of the same IQ, social background, etc) Otherwise, it's junk science.

[star]

professor:

"Numbers in the study are too small, to be statistically significant."

N = 5475. That is not too small. It is larger than the typical number of people used to norm an IQ test.

"There is no mention how these mothers were chosen to be in the study."

This is simply not true. Read the paper.

"They were probably chosen from the same hospital. It's a big sample bias."

No, they were not. What is your reason for injecting obviously false information? You obviously did not read the paper.

"To increase your IQ from 105 to 106 is harder than from 96 to 97."

Do you mind providing us with a reference that supports this assertion? Let me ask you a related one: what evidence can you or anyone produce that shows any method of permanently boosting IQ by even one point?

[Half Sigma]

"Numbers in the study are too small, to be statistically significant."

The Der study, debunking breastfeeding, is DEFINITELY statistically significant. ABSOLUTELY.

The only thing somewhat in doubt is the finding that babies with the GG alleles get a boost in IQ. But that didn't stop the Caspi et all morons from proclaiming they found a huge genetic link between breastfeeding and IQ.

You would think the process to get something published in a scientific journal would be a lot more rigorous than posting something on a blog.

[dearieme]

"You would think the process to get something published in a scientific journal would be a lot more rigorous than posting something on a blog." Not if your conclusion is Politically Correct. Consider Global Warmmongering.

[Half Sigma]

"Not if your conclusion is Politically Correct."

And for some reason, it's politically correct to say good things about breastfeeding.

[dearieme]

It's interesting that breast-feeding overcomes the usual prudishness/puritanism of political correctness. I'm mildly surprised that it hasn't become "mammary nutrient delivery" or the like.

[Pender]

Politically you can't win on this one, HS. If you support breastfeeding, you support the granola-crunching pot-smoking nudist hippies of the all-natural part of the Left. But if you support formula, you support the power-suit-wearing, high-income, blackberry-wielding, stay-at-home-dad-marrying feminist part of the Left.

Is it possible that there might be other benefits to breastfeeding that might enter into the equation? That may seem like blasphemy on this blog, but intelligence is not the only alleged benefit of breastfeeding. Before one concludes that formula (or breastfeeding) is better, wouldn't one want to look at those other alleged benefits?

[Laura]

I wonder how you can conclude that getting up in the middle of the night to heat a bottle of formula is easier? Nursing my kids couldn't have been easier, and sure beat what I saw my sister do with bottles and formula.

[Jody]

Is it possible that there might be other benefits to breastfeeding that might enter into the equation?

There may be, but they weren't the focus of the studies (which were recently in the news) to which HS was responding.

[AllanF]

I, for one, am very impressed. I think this might be his most insightful post ever. Well done H.S.

[fullerene]

Has he not concluded that formula is better? He does say that the "easy way is the best way." I would be reluctant to draw that conclusion on such an infirm basis, particularly when that basis fails to consider other possible effects. Sure, HS only looked at intelligence, but his conclusion goes well beyond that.

Incidentally, a common critcism of the Der, Batty, and Deary study is that it fails to consider dosing. Any breastfeeding for any amount of time is considererd breastfeeding for purposes of the study even if the child was mostly bottle fed for its early life. If there is high dosing variability among those who were breast fed (and based on studies, there appears to be such high variability), the study's conclusion must be reduced significantly.

Compare this approach with a drug study. If a researcher failed to consider dosage in a drug study, his conclusion that the drug showed no or negative correlation with the desired trait would not be considered valid.

Again, I think HS's conclusion is as ill-considered as those he criticizes. Nothing in these two studies leads us to a firm conclusion about the role breastfeeding plays in intelligence.

[Half Sigma]

"Any breastfeeding for any amount of time is considererd breastfeeding for purposes of the study even if the child was mostly bottle fed for its early life. If there is high dosing variability among those who were breast fed (and based on studies, there appears to be such high variability), the study's conclusion must be reduced significantly."

The person who wrote the above has either not bothered to read the study, or not bothered to think much about it, or both.

The study examines sibling pairs in two ways. It compares pairs when one was breastfed and the other wasn't. And it compares pairs where both were breastfed but the duration differed.

In both cases, there is no statistically significant impact on IQ, and also, in both cases, to the extend there is an impact it goes in the OPPOSITE direction, more breastfeeding correlated with lower IQ.

The median duration of breastfeeding is 3 months. That should be ample enough time to demonstrate some sort of effect if there was an effect.

[Rob]

Half, did you do difference including regression to the mean?

I just did for the aggregated Brit and NZ samples using population mean=100 and regression halfway between maternal IQ and population mean and got nonbreastfed costing 1.5 points and breastfeeding giving 2.15 above what we would expect from regression to the mean in the NZ cohort. In the Btitish cohort not breastfeeding cost .95 beyond regression, and breastfeeding gave 1.65 above what regression would predict.

Diddling with heritability, father's IQ and population means will change these, but it still kinda looks like breastfeeding gives a boost and not breastfeeding costs.

[Half Sigma]

When two women with equal measured IQs breastfeed, the breastfeeding women is likely to have a husband with a higher IQ and to be from a higher socioeconomic status.

Remember, children do get half their genes from their father.

Furthermore, I don't see why you think that regression halfway towards the mean is the correct assumption to make, and not the 16%-20% regression found in the British cohort.

The 0% regression towards the mean in the NZ cohort, I agree, shows a positive correlation between breastfeeding and intelligence, but I assume that the causation factor has to do with breastfeeding predicting higher paternal IQ and higher socioeconomic status. And as I pointed out above, the NZ sample size is smaller.

[Dave]

HS, you're way off base to say that formula is easier than breastfeeding. Do you have kids?

Provided you have access to a private space, breastfeeding is a lot more convenient. Unlike formula, it's instantly available, you don't need to mix it or warm it, and you don't need to pack the mixture around with you. If you've ever held a hungry, crying baby, the 2-3 minutes it takes to mix & warm up formula is an eternity.

Breast milk also contains immune-boosting compounds. My 2-month old hasn't had so much as a stuffy nose yet. She is very healthy despite the fact that she's already been on airplanes and held and kissed by lots of people whom we don't insist on sanitizing their hands beforehand.

It's somewhat ironic that lower-income folk don't breastfeed as much. Have you seen how much formula costs? Breastmilk is free.

A final benefit of breastfeeding is the tactile benefits enjoyed by both the baby and her mom.

Who cares if it doesn't make babies smarter - I'm convinced it makes them healthier.

[Dennis Mangan]

Whether or not breastfeeding has any effect on IQ, there's mountains of evidence that it's much healthier for the baby. It has positive effects on immune development, for one. Cow's milk is designed to get a calf to put on about 500 lbs. in a year, and that isn't good for a human being.

[Half Sigma]

"Whether or not breastfeeding has any effect on IQ, there's mountains of evidence that it's much healthier for the baby."

Where? I suspect that most of the evidence is bogus on account of the fact that it fails to take into account that mothers who breastfeed have higher IQ and higher socioeconomic status than mothers who don't.

The evidence is just picking up a correlation between higher socioeconomic status and healthier babies.

[fullerene]

So on the basis of one flawed study, you conclude that all other studies likely make the same mistake?

[Pender]

Rob wrote:

Who cares if it doesn't make babies smarter - I'm convinced it makes them healthier.

Well whatever you're "convinced" of, without actual evidence, you're just making it up.

[Pender]

My mistake, it was Dave who made the comment, not Rob. Sorry Rob.

[Spungen]

Provided you have access to a private space, breastfeeding is a lot more convenient. Unlike formula, it's instantly available,

Dave, it's not always available in the quantities a baby needs or wants. I'm not the only mother I know of who just never seemed to have enough milk. I did my best for six months, but I always had to supplement. Apparently my mother had the same issue. So did my dad's mom.

What was really frustrating was talking to lactaction nurses and La Leche members who insisted the only possible problem was that I just wasn't doing it enough, and I should quit my job and do absolutely nothing else but nurse if that's what it took.

Plus the baby was colicky. Milk of any type aggravated the situation -- I don't care what the breast Nazis say about breast milk being good for colic. Once we switched to Nutramigen, he was a much happier baby. Again, I've known other frustrated mothers in the same position.

Someone mentioned the crunchy granola left, but I got more of a sense this was being driven by "traditional values." It's a good way to justify making women stay home, and a good way for mothers who don't wish to work to justify their decision (baby will end up stupid and sickly if they don't).

Something else to consider is that women who *say* they breastfed for X amount of time may well have been doing what I was doing.

[Rob]

Not a Rob-lem.

HS,

Based on heritability estimates, regression is half-way between the parents IQ and the breeding population IQ. That regression was so low in the British sample means something else is happening.

[Rob]

Sorry to post twice, but I should point out that my regression assumed father's IQ = mothers IQ. In the real world, they correlate, but aren't identical.

Oh, HS, would you email those papers?

[Dennis Mangan]

"Where?"

Have you bothered to look? This is a statement from the American Academy of Pediatrics, found on the Wikipedia page for "Breastfeeding":

"Extensive research, especially in recent years, documents diverse and compelling advantages to infants, mothers, families, and society from breastfeeding and the use of human milk for infant feeding. These include health, nutritional, immunologic, developmental, psychological, social, economic, and environmental benefits."

Of course, HS just refuted the AAP in a blog post.

[Lemmy Caution]

This article compares siblings too:

http://paa2005.princeton.edu/download.aspx?submissionId=50005

It finds no long term benefits to breast feeding except for an IQ benefit. The sample size may be too small though.

[Half Sigma]

The whole "breastfeeding is better" zeitgeist looks to be yet another big scam perpetrated by the medical establishment.

[Gannon]

I have always thought that breastfeeding is essential for the inmune system of the baby.

[bbartlog]

As Rob already pointed out, you need to account for regression to the mean before figuring out whether the kids' IQs are higher or lower than expected. This will be somewhat sensitive to the value you assign to heritability, but as suggested by Rob's numbers, it will give results tending to favor breastfeeding. Having maternal IQ 105, child IQ 104 (breastfed) versus maternal IQ 95, child IQ 97 (non breastfed) looks really good for the formula feeders if you don' account for the regression. But the result flips to favor breastfeeding once you incorporate the regression to the mean.

[Dennis Mangan]

"The whole "breastfeeding is better" zeitgeist looks to be yet another big scam perpetrated by the medical establishment."

So I guess evolution works when it creates racial differences in intelligence, but when it's designing vital nutrition for a developing baby, it's indifferent. Take a biology class, HS.

[Half Sigma]

Rob, bbartlog: Maternal IQ is the most important confounding factor, but there are others. The Caspi authors chose not do do real science by running a regression analysis using multiple factors the way Der did it. I can only assume that if they did things properly, the would be no statistically significant benefit to breastfeeding, because that's what other more rigorous studies have shown.

Dennis Mangan: apparently, the scientists who created baby formula have duplicated human milk close enough so that it's just as good. There doesn't seem to be any solid evidence that modern baby formula is not just as good. Just bullshit evidence which ignores the fact that mothers who breastfeed are smarter, of higher socioeconomic status, and they care more about their babies than women who don't breastfeed.

I suspect that the primary benefit of formula is that there's always enough. Some women seem to have problems producing enough milk.

[Peter]

breast Nazis

Ve haff vays of making you suck!

[Spungen]

I don't see why evolution would necessarily make human milk better for modern infants. All female mammals lactate. People have evolved to be omnivores for a reason. It makes sense that babies would be able to thrive on a variety of foods.

[Mitchsmom]

Hopefully you will have the time to continue such thoughtful, in-depth analysis, and eventually come to a different conclusion about breastfeeding in general based on that. I think it would be hard to do otherwise with much study of human lactation, but it also seems that you have a lot of thoughts on the subject based on your own personal experience, as we all do.

It is in fact considered the preferred choice for infant feeding for many reasons, by the majority of pediatric professionals, by all pediatric (and maternal) professional organizations. I don't think it is a conspiracy. What would the motivation be?

It doesn't take a statistical genius to figure out that breastfeeding your own kind is the way babies are biologically supposed to be fed. It doesn't even take a *human* to figure that out! "Lesser" animals do it everyday. We don't often see a wolf nudging it's infant over to a cow, now do we?

Does that mean that we can't improvise when necessary? Of course not. The wolf baby would die if the mother couldn't provide milk for it. We have been smart enough to figure out a way around that problem.
If you can't or don't want to breastfeed, then so be it. But there is no question that it is the biological norm.

If each mammal's milk isn't best and most fitted for it's species, then why do they differ so vastly? You can find a comparison of some mammals' milks here if interested: http://www.foodsci.uoguelph.ca/dairyedu/intro.html)?

Some sources of information and fun reading if you're into it:
Milk, money, and madness
Breastfeeding: biocultural perspectives
Breastfeeding and Human Lactation
Breastfeeding: a guide for the medical profession

Very interesting topic. Always good to question, keep it up!

[Pender]

Mitchsmom wrote:

It doesn't take a statistical genius to figure out that breastfeeding your own kind is the way babies are biologically supposed to be fed. It doesn't even take a *human* to figure that out! "Lesser" animals do it everyday. We don't often see a wolf nudging it's infant over to a cow, now do we?

Supposed to be? According to whom?

I don't think nature intended us to sit inside in a heated house lit by electric lights as we commune through a liquid-crystal display lit by light-emitting diodes with the rest of humanity over a global telecommunications network either, but here we are.

For that matter, we're not "supposed" to take antibiotics. Do you think those are also therefore bad to you?

How is it any more counterintuitive that man-made formula might be better than the natural alternative than that man-made clothes might be better than running around naked?

[Rob]

Thanks for the papers HS

If your going to look at thousands of people, go to the trouble of giving them all IQ tests, and then following up on them 5-15 years later, wouldn't it be a good effing idea to look at the fathers' IQ scores while your at it, so you know, you have effing decent data by controlling for important sh!t?

Messing around with the data a bit, still using a .5 heritability and regression to 100, to get no effect of breastfeeding on the whole cohorts, we need fathers IQ to be~(I'm too lazy to do algebra, so I just estimated in Excel)
Brit non-breastfed 92.25
Brit breastfed 111.5
NZ non-breastfed 92.5
NZ breastfed 111.5

Are these numbers realistic? Do the husbands(or fathers of children) of lower-IQ women tend to be even lower IQ than the women, and vice versa for higher-IQ moms?

Should I use different population means, or a different heritability?

[Spungen]

Mitchsmom said:
What would the motivation be?

For women who define themselves solely as someone's mom to feel superior, that's what. And to give ammunition to men who want women out of the workforce, or segregated into female jobs where they don't want to compete with them.

[Rich Lawler]

Here's some articles that discuss how breastfeeding transfers immunity from mother to child, suggesting a potential benefit of breastfeeding over formula.

As I understand it (I claim no expertise on the topic), the reason that formula can't be manufactured to duplicate the advantageous immunity-conferring antibodies in breast milk is that each mother's immune system has immunoglobin molecules that are primed to recognize particular pathogens the mother encounters. These immunoglobin molecules can be secreted in breastmilk to the infant, which then coat the mucosal surfaces in the infant's body. To the extent that mom and infant experience the same types of pathogens, which is likely since they are often in close contact, mom's breast milk will confer very specific immunity to the infant. The molecules that mom gives to the infant contain a "immunological memory" so that these molecules can recognize the same pathogens that mom experienced. Immune function is highly individualistic and based on interactions with the environment (only some portions of it are "innate"), thus breastmilk is one of the first environmentally-specific priming agents that the infant's immune system encounters. I suspect that developing a "one size fits all" formula, for example, a formula manufactured in a factory in Switzerland and distributed throughout Europe and Africa, would not be able to confer the environmentally-specific pathogens that mom experienced in her environment.

I am not addressing the IQ breastfeeding linkages, but to me there does seem to be a lot of evidence that breastfeeding does confer particular types of immunity to the infant.

Hanson et al., 2003. The transfer of immunity from mother to child. Immune Mechanisms and Disease, p. 199-206.

Groer et al., 2004. ASsociations between human milk SIgA and maternal immune, infectious, endocrine, and stress variables. J. Hum Lact 20:153-158.

And a splendid overview of human life history theory and immunity:

McDade T, 2003. Life history theory and the immune system: Steps toward a human ecological immunology. Yrbk Phys Anthropol 46: 100-125.

[Joe]

Purely anecdotal, but my four children would have died without formula. The first refused to even try to breastfeed. The next two gave up on their own after a few feedings. The last breastfed, but she started losing weight and we had to supplement her diet with formula (the woman from the La Leche league said nasty things to us for that--I'll always hold that group in contempt for their refusal to put life before their precious theories.)

Also, I believe I read an article recently debunking many of the immunity claims of breastfeeding, but I don't have a reference. Regardless, the value of good nutrition surely outweighs malnutrition with any possible breast milk immunity.

[Daniel]

Strange that both the above blog posting as the meta-analysis of Geoff Der, G. David Batty, and Ian J. Deary did not mention the best study so far on this topic: "the association between duration of breastfeeding and adult intellengence" of Mortensen et al in the JAMA,2004, vol 217, no 18. They found a very clear association between duration of breastfeeding and intelligence after correcting for confounding factors like education status of the parents and social status. Strange that this study is not included or even mentioned in a meta-analysis.

[Half Sigma]

In response to Daniel above, the Der study said that studies which include socioeconomic factors, but don't test the mother's IQ, ignore the most important confounding factor of all.

The meta-analysis shows that as more confounding factors are added to the analysis, the lower the association between breastfeeding and IQ.

Between sibling pairs where one was breastfed longer, Der found NO DIFFERENCE in IQ (in fact, a slightlylower IQ for the child breastfed longer).

[SteveBrooklineMA]

Thanks a million for your analysis. At a breast-feeding class yesterday, the instructor told us that breast-fed babies have a 7 point higher IQ. My right eyebrow rose in skeptical reflex reaction. A little googling this morning led me to the two papers you discuss, obviously at odds with each other. A little more googling, and I found that you have saved me a lot of trouble. Nice work! I'll be sure to return to halfsigma.