 The Golden Age ended when American superhero book sales declined, leading to the cancellation of many characters; but Superman was one of the few superhero franchises that survived this decline, and his sustained popularity into the late 1950s helped the second flourishing in the Silver Age of Comic Books, when characters such as Spider-Man, Iron Man, and The X-Men were created.After World War 2, American superhero fiction entered Japanese culture.Astro Boy, first published in 1952, was inspired by Mighty Mouse, which itself was a parody of Superman.