The Hypodermic needle effect is a theory that suggests the media in a film or a computer game is capable of manipulating or convincing the brain into believing that a certain aspect of the media is real. This causes the audience to believe what they see or hear in films or computer games.
This isn't always a good thing. In 1993, in Liverpool, a young 3 year old boy, James Bulger was sadly, abducted, tortured and murdered. The two boys found guilty of his murder were only kids themselves. Robert Thompson and Jon Venables who were both aged 10, suposedly saw the film "Child's Play 3" and were attempting to re-create a scene they had saw. This is a case of the Hypothermic Needle Theory.