It might be a sign of better nutrition during childhood. Things like iron from 6 months and up have a pretty big effect on brain function later in life.
And if this was studied and proven to be true then all you contrarians would again reverse the arrow of causation saying that "no no no, it is not the iron supplements it is X".
Yes, of course we can not ethically proof that any one supplement has some precise effect, but instead of burying your head in the sand you can follow some general trends.
> And if this was studied and proven to be true then all you contrarians would again reverse the arrow of causation saying that "no no no, it is not the iron supplements it is X".
The point here is that this study didn't prove anything - it's an observational study. The results are consistent with a range of possibilities, not just the ones the study authors were the most interested in promoting to the public. Academic guesses about causation are often wrong, and they're often biased towards what's interesting to academics and what raises the profile of their research. So it's important to highlight other possibilities, especially the "boring" ones that wouldn't get as much attention for the study authors.
There are high-quality studies that actually prove things. For example, we know the impact of lead on IQ because of direct interventions (children who have lead above a certain threshold undergo chelation therapy, so you can compare those right below to those right above).
Source?