Clarify the results of the breast milk is best data. Debunk the myth that the differences in outcome came from socioeconomic differences in the groups.

What the evidence says

You're right to question the data, but the science doesn't actually let us call socioeconomic confounding a myth. It's the opposite: some of the best available studies show it's a real and important part of the story.

A well-known method for testing this is a sibling-comparison study. Researchers look at brothers and sisters from the same family, where one was breastfed longer than the other. This cancels out most of the family background, income and parenting style differences at once, since siblings share those things. One large U.S. study using this method found that once you compare siblings this way, breastfeeding was no longer linked to better behaviour or lower obesity, and the link to cognitive test scores also became much weaker or disappeared for most measures.

A separate systematic review looking specifically at brain development studies found that a large share of the reported IQ boost from breastfeeding could be explained by confounding, mainly the mother's education, income and her own IQ. The review's authors felt this pattern was consistent enough that they didn't expect future studies to change the picture much for cognitive outcomes.

That said, it isn't confounding all the way down. Studies that focus on infections, like ear infections, stomach bugs and respiratory illness, tend to hold up better even after accounting for family background, and there's a clear biological reason: breast milk carries antibodies and immune factors. Some newer sibling studies from Israel and Japan also found breastfeeding still linked to fewer developmental delays even after accounting for shared family factors, so this part of the debate isn't fully closed either.

The honest bottom line for a parent: breastfeeding has real, biologically plausible benefits, particularly around infections in infancy, and Health Canada continues to recommend it. But some of the bigger claims about IQ, behaviour and long-term outcomes are probably inflated by the fact that mothers who breastfeed longer also tend to have more education, income and support, not because formula-fed babies are worse off.