In 1968, Gerbner conducted a survey to demonstrate this theory. Following his previous results, he placed television viewers into three categories: light viewers (less than 2 hours a day), medium viewers (2–4 hours a day), and heavy viewers (more than 4 hours a day). He found that heavy viewers held beliefs and opinions similar to those portrayed on television, which demonstrated the compound effect of media influence. Heavy viewers experienced shyness, loneliness, and depression much more than those who watched less often. From this study, Gerbner began work on what would become the Mean World Index, which subscribes to the notion that the heavy consumption of violence-related content leads the viewer to believe the world is more dangerous than it actually is.