I think this is one thing that would be very, very difficult to study right. There is huge selection bias. Playing chess for a long time select people who are

I think this is one thing that would be very, very difficult to study right. There is huge selection bias. Playing chess for a long time select people who are very different from general public and from people who just decided to play but will drop it soon. People also play chess in a different way: some people play just to kill time while some play really competitively. I think that regardless of whether chess in itself doesn't do much (I would be surprised if that was so), people who can stick to playing chess competitively are more likely to also exercise their minds in other ways. I also believe that playing chess is one of those exercises that have other beneficial results (like training focus). Other example might be running which I think of the beneficial effects one that people forget to mention is training ones willpower (assuming you do more than just light jog).