According to research in anomalistic psychology, claims of poltergeist activity can be explained by psychological factors such as illusion, memory lapses, and wishful thinking. A study (Lange and Houran, 1998) wrote that poltergeist experiences are delusions "resulting from the affective and cognitive dynamics of percipients' interpretation of ambiguous stimuli". Psychologist Donovan Rawcliffe has written that almost all poltergeist cases that have been investigated turned out to be based on trickery, whilst the rest are attributable to psychological factors such as hallucinations.