Real Life Joker's

[Jumpin' Jack]

This is an interesting thing and maybe quite a lot of you know about it. I think nolan was inspired about this real life thing.

A Glasgow smile (also known as an Anna grin, Brazilian Smile, Chelsea grin or Chelsea smile) is a nickname for the malicious practice of cutting a victim's face from the edges of the mouth to the ears, often using a credit card to hold the mouth open in modern times: the cut - or its scars - form an "extension" of what resembles a smile. Sometimes to further hurt or even kill the victim, he or she would then be stabbed or kicked, most notably in the stomach (or in case of kicking, the groin), so that the face would be ripped apart when the victim screamed. The practice originated in the Scottish city of Glasgow, which gave it its name. It also became popular in Chelsea, London (where it is known as a "Chelsea grin") and other areas of Britain, for gangs hoping to leave a message to rival gang members. If cut deep enough, the victim can likely bleed to death.
In the murder of the Black Dahlia, it was noted that the body was given a Glasgow smile, which also could have played a part in her death along with her body being cut into two.

 

[Bathead]

Well the original inspiration for the Joker in the comics was Conrad Veidt in "The Man Who Laughs"
<<<<(see my colorized avatar for the pic) ,
a movie based on Victor Hugo's story about a man who was mutilated that way as a child.