A very interesting theory about The Joker is that he isn't actually a person - he's a virus (or even some kind of spirit/ghost) that infects people with The Joker persona. It actually makes quite a lot of sense, as the origins of the character have never been truly pinned down. Therefore, it could be that the individuals who have portrayed the Clown Prince of Crime due to a potential infection/possession either have no real origin to speak of (they were simply infected/possessed) or their different origins simply end up contrasting with each other because they're all different people. Interestingly, something similar to this theory has actually happen in Batman Beyond: Return of The Joker. A microchip containing his consciousness was made after his death which, in the case of that particular story, was implanted in the head of an ageing Tim Drake.
