Stuart Ritchie goes through the research with a hard eye. This is one of those things that some people are really invested in being true, exactly the situation that can result in publication bias (the studies that show a null hypothesis or even negative result are more likely to be abandoned, rejected for publication, or referenced less often), as well as a perception bias, because journalists like this idea a great deal and love to puff up the claims when the report on it, so the the general public gets the idea that a positive effect has been proved over and over.
Yet when one looks at the data and presses it hard, it turns out to be squishy. It is interesting to watch Ritchie follow whatever trails he can find and see where they actually do go, regardless of what the signs say. We were very pro breastfeeding in this house when the oldest two were born 40 years ago. It was a transition time, from our mothers' era that only poor women breastfed - or maybe some nurses or nature fanatics - to a time when all the best, educated, modern aware young mothers who had "done the research" as we say decided that the breast was better. We knew one woman who nursed her daughter until she was three.
Ritchie does not comment on other claimed benefits, which he considers quite possible, such as bonding or immunity. Those things are hard to measure, and IQ sits out that as a generally measurable item, and thus useful for research to learn more general lessons.
His eventual conclusion is that it is still possible that there is some intelligence advantage, but most of what is claimed can be eroded down to a point where if there is an effect, it is small.
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