a credible witness before the event. The time interval between the dream and the event should be short. The event should be unexpected at the time of the dream. The description should be of an event destined literally, and not symbolically, to happen. The details of dream and event should tally. David Ryback, a psychologist in Atlanta, used a questionnaire survey approach to investigate precognitive dreaming in college students during the 1980s. His survey of over 433 participants showed that 290 or 66.9 percent reported some form of paranormal dream. He rejected many of these reports, but claimed that 8.8 percent of the population was having actual precognitive dreams. 21st century In 2011 the psychologist Daryl Bem, a Professor Emeritus at Cornell University, published findings showing statistical evidence for precognition in the Journal of Personality and Social Psychology. The paper was heavily criticised and the criticism widened to include the journal itself and the validity of the peer review process. In 2012, an independent attempt to reproduce Bem's results was published, but it failed to do so. The widespread controversy led to calls for improvements in practice and for more research. Scientific reception Claims of precognition are, like any other claims, open to scientific criticism. However, the nature of the criticism must adapt to the nature of the claim. Pseudoscience Claims of precognition are criticised on three main grounds: There is no known scientific mechanism which would allow precognition. It breaks temporal causality, in that the precognised event causes an effect in the subject prior to the event itself. The large body of experimental work has produced no accepted scientific evidence that precognition exists. The large body of anecdotal evidence can be explained by alternative psychological mechanisms. Consequently, precognition is widely considered to be pseudoscience.Zusne, Leonard; Jones, Warren H. (1989). Anomalistic Psychology: A Study of Magical Thinking. Lawrence Erlbaum Associates, Inc. p. 151. Violation of causality Precognition would violate the principle of antecedence (causality), that an effect does not happen before its cause. Information passing backwards in time (retrocausality) would need to be carried by physical particles doing the same. Experimental evidence from high-energy physics suggests that this cannot happen. There is therefore no direct justification for precognition from physics. Precognition would therefore also contradict "most of the neuroscience and psychology literature, from electrophysiology and neuroimaging to temporal effects found in psychophysical research." Lack of evidence A great deal of