Originally Posted by milost
Most of them all took the plunge into the vat of chemicals though. Well, until recently. I'm also pretty sure the Killing Joke origin of him with his pregnant wife is true with the flashback. Other people in the story like Batman and Gordon don't know that though of course, only the Joker. Hence his great "multiple choice" line to Batman. I do think he was originally that comedian that was forced into the heist who lost his wife and child and had his skin bleached in a vat of chemicals.
There is no solid proof that any of the Joker's origins is true. Even the Joker himself doesn't know how he came to be hence the "multiple choice" line.