Poltergeist disturbances are characterized by a series of short-lived anomalous physical occurrences (e.g., objects sporadically moving about on their own, without any force apparently acting upon them) which have been traditionally attributed to the mischievous acts of a spirit, demon, or some other kind of discarnate entity. However, there are several parapsychological findings which seem to indicate that poltergeists are a human-related phenomenon, rather than a spirit-related one (for a concise overview, see this Adobe PDF slideshow). One of these findings is the rather frequent observation that poltergeist disturbances have a tendency to occur in the presence of a certain person (often referred to as the poltergeist agent). This suggests that the disturbances may reflect a form of psychokinesis (PK, or "mind over matter") on the part of the agent, which acts largely on the unconscious level.
In certain rare instances, parapsychologists have had the opportunity to explore this possibility further by bringing poltergeist agents into the laboratory and testing them for PK under controlled conditions. An article which describes two of these efforts made by parapsychologists can be found here. (NOTE: the article is in Adobe PDF format)