When Batman was first introduced into comics he was a pretty serious character. What I mean is, there was adult content. The villains would kill people, sometimes in very gruesome ways, like the Joker and his laughing gas, for instance. Heck, even Batman killed people. However, when did they tone-down the comics, how many appearences did the Joker have before he became a practical joker (no pun intended), and when did the comics become serious again? Thanks in advance.