The 1980's were a period of drastic change for comic books. Whereas in the previous decades, comics had struggled with their cartoonish and childlike manner, during the 80's they turned towards a more dark and mature route. The change occurred for several reasons, but one of the most important was that this was used to address changes in readership during the later part of the cemetery. Comics tried to become more serious, and they succeeded to a great extent, particularly once they were free of the Comics Code. Overall, the 1980's more than any other decade saw a change in the way comics were perceived, both by the creators and the readers.
Superheroes in the 1980's became dark because the creators of the comics were trying to appeal to a more mature group of readers. With the creation of the comic shop and the slow death of the comic's popularity among children, the fan base that was purchasing comics was slowly becoming older. This meant that if comics didn't change from their campy and foolish dialogue that was established during the 1950's, they would lose this fan base. As a result superhero comics became darker as a way of becoming more mature and appealing to a broader range of adult and young adult audiences.
Perhaps the best example of the gradual darkening of comics can be seen in the Batman stories that came out of this period. Batman: Year One and The Dark Knight Returns were both a drastic departure from the sheer ridiculousness of the previous Batman comics. The 80's saw Batman returning to his darker roots, with his character becoming more of the grim, obsessive shadow of terror that people today know and love. Batman's rogue gallery, also, became more serious, for there are few today that would contest the utter insanity of everyones favorite psychotic clown, The Joker, or the consuming fear controlled by Scarecrow. However, Batman was not the only character getting a little rougher around the edges. Heroes like Superman and Spiderman were also having their stories become darker. The result of this was that they often reflected a great deal more of the psychological content to the characters, something older readers would appreciate but that would've been completely lost on younger fans.
The gradual darkening that took place in superhero comics in the 1980's was the result of societal changes. Though a large part of this change was spurred by the increasing age of the readership, the direction of the change was dictated by the time. The 80's was a generation dealing with the fall out of the war in Vietnam, as well as the continuation of the Cold War. There was a great deal of skepticism and little faith in political leaders, as a result of the Watergate Affair. It was a period of general pessimism towards the state of the world at large, and this was reflected in what people were choosing to read, particularly the many distopian or apocalyptic stories that came about, such as Crisis on Infinite Earths. Overall, the 1980's saw much of society grow up and look at the world much more starkly, and comics was no exception.