Alleged poltergeist phenomena cause fear, and their unsatisfactory explanation perpetuates superstitious ideas, such as houses which are said to be haunted. Houses which have experienced poltergeist knockings or transport of objects, are shunned, causing losses of property values and the movement of families from such residences. Poltergeists are believed to be spirits, usually mischievous and occasionally malevolent, that manifest by moving objects (stones, dirt, and other small objects) making noises and vile smells, and attacking people and animals. It has also been argued that the phenomenon may in some cases be due to sub-conscious psychokinesis on the part of an individual in the building, usually a child or teenager with unresolved emotional tensions. Such phenomena may last for several hours to several months.
Disturbances by poltergeists are reported in over 100 countries around the world. A study of some 500 reported cases dating from 1800 was undertaken in the 1970s.