It's really not a theory it's the truth and well known. Of course some lefties are better with their right than others but if you go out on the street and pick 10 righties and ten lefties at random and make them use their off hand to do a variety of tasks the lefties will do far better than the righties.

Back in the old days and in many cultures to this day lefthanded people are seen as evil or bad and these folks in school are forced to use their right hands for everything.