On Earth-1 the Joker was a unnamed criminal who led a gang under the guise of the Red Hood. When Batman and Robin perused the criminals at the Monarch Playing Card Company, the Red Hood fell into a vat of chemicals. After he was retrieved an unmasked, the police learned he had been instantly transformed into the Monarch of Manace. (Detective Comics #168, February 1951)
      Batman and the Joker engaged in numerous confrontations through the decade-plus struggle. For each crime thwarted, the Joker managed to get away with other offences as he veered between the comical and deadly, all the while attempting to get to get the best of the Caped Crusader. (Batman#251, September 1973)
      The Joker continued to fine new and innovative ways to torment the world, including the time he used his venom to supply fish in the Atlantic Ocean with his trademark grin. He then applied to copyright the Joker-fish, intending to profit from the royalty fishermen and restaurants would have to pay him. (Detective Comics #475-476, April 1978/June 1978)
      In the wake of the Crisis on Infinite Earths, the Harlequin of Hate's origins became cloudy. It was thought he might be a chemical-engineer-turned-failed-comedian who got roped into committing the robbery as the Red Hood in order to earn money to care for his pregnant wife. A freak lightning strike killed his wife, but despite his pain the same day. In this version, Batman was not present when the man fell into the vat of chemicals at the Ace Chemical Processing Plant, swimming through a pipe and emerging in the polluted  river disfigured. (Batman: The killing Joke , 1988) The version was mostly substantiated by an eyewitness to the crime, the Riddler. (Batman: Gotham Knights #54, August 2004) 
       The exact nature of the man's mental illness was subject to debate among professionals. To the Joker, his new incarnation meant he was a blank slate, constantly reinventing himself-as befit his physical looks- while he sought a core personality. (Batman #663, April 2007) Others have suggested that his insanity was actually some advanced form of sanity that allowed him to see the world in a way no one else could comprehend. (Arkham Asylum,1989) It was made demonstrated that the Joker feared nothing; even the Scarecrow's fear gas failed to conjure up nightmare images.