I think this is a common sense thing. The roles and duties are printed on both the booking and check-in screen and on the exit row briefing card. One must analyze their own status and abilities and determine in a crisis/emergency situation, would they be able to execute the instructions (which include opeating the latch mechanism and lifting a 40 lb. door) without endangering others. Yes, I have never had to evacuate an aircraft in an emercency situation, but I have been in other stressful situations and I didn't suffer a panic attack or curl up into a ball. I understand English, can follow instructions, do not travel with children under 15, can read and understand how to open a door, and I can lift a weight aproximate to that of an exit door. So, I sit in exit rows. If any of the above were not true (let's say I broke my arm or was travelling with my 10 year old niece), I would not.