Mean world syndrome is a term coined by George Gerbner to describe a phenomenon whereby violence-related content of mass media makes viewers believe that the world is more dangerous than it actually is. Mean world syndrome is one of the main conclusions of cultivation theory. Gerbner, a pioneer researcher on the effects of television on society, argued that people who watch television tended to think of the world as an intimidating and unforgiving place. A direct correlation between the amount of television one watches and the amount of fear one harbors about the world has been proven, although the direction of causality remains debatable in that persons fearful of the world may be more likely to retreat from it and in turn spend more time in indoor, solitary activity such as television watching.

The number of opinions, images, and attitudes that viewers tend to form when watching television will have a direct influence on how the viewer perceives the real world. They will reflect and refer to the most common images or recurrent messages thought to affect their own real lives. Gerbner once said: "You know, who tells the stories of a culture really governs human behaviour. It used to be the parent, the school, the church, the community. Now it's a handful of global conglomerates that have nothing to tell, but a great deal to sell."

Gerbner says that the spread of the syndrome has become more intense over time. He describes how newer technologies such as VCR, DVD, and cable do not disturb the cultivation theory, but actually allow more complete access and spread of recurrent messages, although widening access to the Internet world of information can counteract that. The 1930s behaviorism models, the Payne Fund Studies, show that the effect that mass media has on our behavior is considerable. This is called the hypodermic needle model: people are injected with appropriate messages and ideas constructed by the mass media.