Five schools of thought to explain UFO sightings: * Some UFOs are spacecraft used by extraterrestrials to visit Earth. * According to the Psychosocial Hypothesis, the UFO craze is a modern social phenomenon similar to reports of witches in past centuries. * The Psychopathological hypothesis suggests that some people satisfy personal psychological needs by pretending to have been abducted by aliens. * Various occult, paranormal, supernatural or religious explanations. * UFO's are best explained as advanced, secret or experimental aircraft of decidely earthly origin.