Originally conceived as an evil "court-jester" type, the character was initially rejected by studio writer Bill Finger as being "too clownish," but he later relayed the idea to Bob Kane. Kane, who started out as a gag artist, loved the concept and encouraged its production. Finger found a photograph of actor Conrad Veidt wearing make-up for the silent film The Man Who Laughs, and it was from this photograph that the Joker was modeled. This influence was referenced in the graphic novel Batman: The Man Who Laughs, a retelling of the first Joker story from 1940.
In his initial dozen or so appearances, starting with Batman #1 (1940), the Joker was a straightforward spree killer/mass murderer, with a bizarre appearance modeled after the symbol of the Joker known from playing cards. It is of note that in his second appearance he was actually slated to be killed off, with the final page detailing the villain accidentally stabbing himself, lying dead as Batman and Robin run off into the night. DC editor Whitney Ellsworth thought the Joker was too good a character to kill off, suggesting that he be spared. A hastily drawn panel, calculated to imply that the Joker was still alive, was subsequently added to the comic.
For the next several appearances, the Joker often escaped capture but suffered an apparent death (falling off a cliff, being caught in a burning building, etc.), from which his body was not recovered. In these first dozen adventures, the Joker killed close to three dozen people, impressive for a villain who didn't use giant robots, mutant monsters, or space lasers, as was the status quo between 1940 until around 1942. Ironically, the turning point came in "Joker Walks the Last Mile" (Detective Comics #64), when the Joker was actually executed in the electric chair only to be chemically revived by henchmen.
While the Joker was back, he was decidedly less deadly than previous engagements. At this point, the editors decided that only one-shot villains should commit murder, so as to not make Batman look impotent in his inability to punish such recurring foes as the Joker or the Penguin. As the Batman comics softened their tone, the Joker shifted towards a harmless, cackling nuisance. He quickly became the most popular villain and was used frequently during the Golden Age of Comic Books. The use of the character lessened somewhat by the late 1950s, and disappeared almost entirely when Julius Schwartz took over editorship of the Batman comics in 1964.
In 1973, the character was revived and profoundly revised in the Batman comic stories by Dennis O'Neil and Neal Adams. Beginning in Batman #251, with the story "The Joker's Five Way Revenge", the Joker becomes a homicidal maniac who casually murders people on a whim, while enjoying battles of wits with Batman. This take on the character has taken prominence since. Steve Englehart, in his short but well-received run on the book, added elements deepening the severity of the Joker's insanity.
Joker even had his own nine-issue series during the 1970s in which he faces off against a variety of foes, both superheroes and supervillains. Although he was the protagonist of the series, certain issues feature just as much murder as those in which he was the antagonist; of the nine issues, he commits murder in seven. The development of the Joker as a sociopath continues with the issues "A Death in the Family" (in which readers voted for the character to kill off Jason Todd) and The Killing Joke in 1988, redefining the character for DC's Modern Age after the company wide reboot following Crisis on Infinite Earths.
A major addition to the character was the introduction of the character Harley Quinn. Originally introduced in Batman: The Animated Series, Quinn is a clinical psychiatrist who falls hopelessly in love with the Joker in Arkham Asylum after he relays his tale of having an abusive father and a runaway mother, and now serves as his loyal, if daffy, sidekick, costumed in a skintight harlequin suit. Their relationship often resembles that of an abusive domestic relationship, with the Joker insulting, hurting, or even attempting to kill Quinn, who remains undaunted in her devotion. She was popular enough to be integrated into the comics in 1999 and a modified version of the character (less goofy, but still criminally insane and utterly committed to the Joker) was also featured on the short-lived live-action TV series Birds of Prey.