The cultivation theory, developed by George Grubner, is a social theory that basically states the more a person watches or “lives” in a television world, the more they are to believe the social reality depicted by television. Usually, the reality depicted is a harsh, negative message; thus, the idea behind the ‘Mean World Syndrome’.

Here is an example from a survey taken in New Jersey:

In a survey of about 450 New Jersey schoolchildren, 73 percent of heavy viewers compared to 62 percent of light viewers gave the TV answer to a question asking them to estimate the number of people involved in violence in a typical week. The same survey showed that children who were heavy viewers were more fearful about walking alone in a city at night. They also overestimated the number of people who commit serious crimes. This effect is called ‘mean world syndrome’. One controlled experiment addressed the issue of cause and effect, manipulating the viewing of American college students to create heavy- and light-viewing groups. After 6 weeks of controlled viewing, heavy viewers of action-adventure programs were indeed found to be more fearful of life in the everyday world than were light viewers.

http://www.utwente.nl/cw/theorieenoverzicht/Theory%20Clusters/Media,%20Culture%20and%20Society/Cultivation_Theory-1/