More weight may equal less brainpower, study finds

I would suggest that it is most likely the writing of this article that has drawn an invalid conclusion. Without reading the study results paper itself, we can't say that the study was at fault. However, most reporters wouldn't make the distinction

I agree with VerityBrown - correlation is not causation - but it goes beyond thyroid problems, etc. that might cause weight gain. One has to have a desire to be intellectually challenged - and it's never too late.

This sounds like a badly designed study. Correlation is not causation! It doesn't sound as if they took any other factors into account, such as diseases (like thyroid problems) that might pack on pounds AND cause cognitive impairment.