My son is has done the same thing! He is my only child right now, and at about a year and a half he started hitting himself in the head - when he would get upset. It really threw me for a loop. I tell him "no baby, don't hit yourself" and he has slowly stopped doing it. He is 27 months right now and I have only seen him do it once in the past 4-5 weeks, which is a big improvement from the daily whaps he would give himself. It seemed like it was his way of punishing himself. He would usually do it when I was saying no to him, no you can't have that, or no put that down... I am also interested in finding out more about this behavior.