Poltergeists are said to bother only one person at a time. This person is called their "focus". Poltergeists will bother only their focus. They are said to only last a few weeks, then will move on to another target. Often poltergeists are reported around children and adolescents. Some scientists think that poltergeists only exist in a person's head as a result of psychokinesis. They think that the "poltergeist effect" is a result of mental trauma.