October 25, 2015

Breastfeeding and IQ

Breastfeeding is VERY politically correct these days.  Mothers who do not breastfeed can be harassed by other mothers over it.  Why?  Because breastfeeding is thought to be  "more natural" and hence better for the baby.  But better in what way? One claim is that is helps the child's IQ.  But the studies have not been very supportive of that. So the latest very extensive study is of great interest.  Abstract below:

Breastfeeding and IQ Growth from Toddlerhood through Adolescence

By Sophie von Stumm &  Robert Plomin

Objectives
The benefits of breastfeeding for cognitive development continue to be hotly debated but are yet to be supported by conclusive empirical evidence.

Methods
We used here a latent growth curve modeling approach to test the association of breastfeeding with IQ growth trajectories, which allows differentiating the variance in the IQ starting point in early life from variance in IQ gains that occur later in childhood through adolescence. Breastfeeding (yes/ no) was modeled as a direct predictor of three IQ latent growth factors (i.e. intercept, slope and quadratic term) and adjusted for the covariates socioeconomic status, mother's age at birth and gestational stage. Data came from the Twins Early Development Study (TEDS), a prospective cohort study of twins born between
1996 and 1994 in the United Kingdom, who were assessed 9 times on IQ between age 2 and 16 years (N = 11,582).

Results
Having been breastfed was associated with a small yet significant advantage in IQ at age 2 in girls (? = .07, CI 95% from 0.64 to 3.01; N = 3,035) but not in boys (? = .04, CI 95% from -0.14 to 2.41). Having been breastfeeding was neither associated with the other IQ growth factors in girls (slope: ? = .02, CI 95% from -0.25 to 0.43; quadratic: ? = .01, CI 95% from -0.02 to 0.02) nor in boys (slope: ? = .02, CI 95% from -0.30 to 0.47; quadratic: ? = -.01, CI 95% from -0.01 to 0.01).

Conclusions
Breastfeeding has little benefit for early life intelligence and cognitive growth from toddlerhood through adolescence.

Von Stumm S, Plomin R (2015). Breastfeeding and IQ Growth from Toddlerhood through Adolescence. PLoS ONE 10(9): e0138676. doi:10.1371/journal.pone.0138676

The study is persuasive rather than conclusive.  I think IQ of the mother should have been controlled for.  I made the same criticism of a noted Brazilian study which did find some benefit from breastfeeding.

Another concern is that the measures of IQ used at different ages were not well correlated. They could obviously not be the same but correlations between them as low as .18 are a serious concern.

Overall, however, the general agreement of the studies on the matter leads me to agree that breastfeeding has no effect on IQ.  It may however have other benefits.


UPDATE:  Those .18 correlations in Table 1 are of course absolutely appalling so I have been thinking about that.  The simple thing to say is that the questions you ask a 4-year-old to assess his IQ and the questions you ask a 16-year old to assess his IQ are necessarily  very different -- so a high correlation is not to be expected.  There is however a conventional solution to that conundrum:  Use a spiral omnibus test -- where the questions start out very easy and gradually get harder.

The authors above, apparently, did not however have that luxury.  So their solution was a creative one which I rather admire.  They took the first eigenvector of the battery they did have and standardized that as IQ (mean 100; SD 15).

So what do we find from that?  It could be argued that they have for the first time made IQ tests that are valid for particular age groups.  And in that case what we see is that IQ is very variable  throughout the lifespan.  Being bright at 2 tells us little about  IQ at 16

And I think that is an important finding.  In particular it conforms to other findings that environment is important in early life but, as time goes by it is the genetic given that manifests itself.

Be that as it may, the measures of IQ used in the early years are clearly just not valid.  They do not correlate with well-accepted  measures from later life. Putting it more bluntly, trying to measure IQ at age 2 is just a no-go.  It fails.  It tells you  nothing.

In that case the slight effect seen at age 2 is a nonsense and not to be taken seriously.

And Table 1 in the article has another interesting implication.  It bears on the "Eleven Plus" exam used in England to filter access to Grammar (selective) schools. There was no IQ given for age 11 but there was for age 12.  And we see there that  the correlations for age 12 and up averaged around .6.  That is not ideal but, given changes in IQ throughout the early lifetime, is probably as good as can be expected. Those eigenvectors were not too bad as IQ measures!






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