Works in this genre may be considered adult for any number of reasons, such as complex themes, sexual or suggestive content, graphic violence, profane language or dark humor. Works in this genre may explore philosophical, political, or social issues. In the United States, before the enforcement of the Hays Code , some cartoon shorts contained humor that was aimed at adult audience members rather than children. Following the introduction of the Motion Picture Association of America film rating system , independent animation producers attempted to establish an alternative to mainstream animation. Initially, few animation studios in the United States attempted to produce animation for adult audiences, but later examples of animation produced for adults would gain mainstream attention and success.