Supply and demand is also influenced by ability to become qualified. I'm not qualified to flip burgers, but any fast food place could get me qualified in just a few hours. I'm not qualified to be a doctor and it would take me years of training to get that qualification. I've met people who failed to qualify as a burger flipper - you can correctly guess by that statement they are very disabled, I've met many who failed to qualify as a doctor, and all are smart people since people who are not wouldn't even try.