Recent research suggests a connection between ADHD and anomalies in the brain’s reward system related to the neural circuits of motivation and gratification, which would explain why kids with ADHD have trouble motivating themselves, and they would experience less gratification upon completion:
http://www.sciencedaily.com/releases/2010/02/100203084302.htm
http://www.medpagetoday.com/Pediatrics/ADHD-ADD/15872
http://healthomg.com/2011/03/28/adhd-brain-may-be-a-little-different/
