The positive effects of chess may be higher, but I presume that the average rationalist or LWer practices 8 hours of abstract reasoning a day, simply by doing their job. Let us think about Bobby Fischer. He probably practiced at least 10 hours a day - maybe then another hour of chess did not have an impact as positive as an hour of tennis, swimming etc. At least, he did not think so.

The situation is of course very different, if you are a professional athlete. Then some hours of chess in the free time is (probably) a better way to train your brain, but so would be reading a book about AI, rationality, etc.

All I am saying is that the time you can improve your mental abilities by thinking about some hard problems is limited and above a certain threshold (maybe 8h a day, maybe a lot more or less depending on the kind of activities, the specific person etc.) it might be better to do something else, like sleep, go for a walk, listen to music or engage in some physical activity.

Here is some further evidence that physical activity might have a positive impact on your brain: (I neither have the time nor the competence to evaluate the quality of these papers; also I hope that they're visible from outside a university network)

Cotman, Carl W.; Engesser-Cesar, Christie: Exercise Enhances and Protects Brain Function. http://journals.lww.com/acsm-essr/Abstract/2002/04000/Exercise_Enhances_and_Protects_Brain_Function.6.aspx

Cotman, Carl W. , Berchtold, Nicole C., Christie, Lori-Ann: Exercise builds brain health: key roles of growth factor cascades and inflammation. http://www.sciencedirect.com/science/article/pii/S0166223607001786#

Colcombe, Stanley J., Erickson, Kirk I., Raz, Naftali, Webb, Andrew G., Cohen, Neal J., McAuley, Edward, Kramer, Arthur F.: Aerobic Fitness Reduces Brain Tissue Loss in Aging Humans. http://biomedgerontology.oxfordjournals.org/content/58/2/M176.short

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