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Extrasensory perception (ESP) involves reception of information not gained through the recognized physical senses but sensed with the mind. The term was adopted by Duke University psychologist J. B. Rhine to denote psychic abilities such as telepathy, clairaudience, and clairvoyance, and their trans-temporal operation as precognition or retrocognition. ESP is also sometimes referred to as a sixth sense. The term implies acquisition of information by means external to the basic limiting assumptions of science, such as that organisms can only receive information from the past to the present.
Parapsychology is the study of paranormal psychic phenomena, including ESP. Parapsychologists generally regard such tests as the ganzfeld experiment as providing compelling evidence for the existence of ESP. The scientific community rejects ESP due to the absence of an evidence base, the lack of a theory which would explain ESP, the lack of experimental techniques which can provide reliably positive results, and considers ESP a pseudoscience.
In the 1930s, at Duke University in North Carolina J. B. Rhine and his wife Louisa tried to develop psychical research into an experimental science. To avoid the connotations of hauntings and the seance room, they renamed it "parapsychology". While Louisa Rhine concentrated on collecting accounts of spontaneous cases, J. B. Rhine worked largely in the laboratory, carefully defining terms such as ESP and psi and designing experiments to test them. A simple set of cards was developed, originally called Zener cards - now called ESP cards. They bear the symbols circle, square, wavy lines, cross, and star; there are five cards of each in a pack of 25.
In a telepathy experiment, the "sender" looks at a series of cards while the "receiver" guesses the symbols. To try to observe clairvoyance, the pack of cards is hidden from everyone while the receiver guesses. To try to observe precognition, the order of the cards is determined after the guesses are made.
In all such experiments order of the cards must be random so that hits are not obtained through systematic biases or prior knowledge. At first the cards were shuffled by hand, then by machine. Later, random number tables were used, nowadays, computers. An advantage of ESP cards is that statistics can easily be applied to determine whether the number of hits obtained is higher than would be expected by chance. Rhine used ordinary people as subjects and claimed that, on average, they did significantly better than chance expectation. Later he used dice to test for psychokinesis and also claimed results that were better than chance.
In 1940, Rhine, J.G. Pratt, and others at Duke authored a review of all card-guessing experiments conducted internationally since 1882 titled Extra-Sensory Perception After Sixty Years. It included details of replications of Rhine's studies. Through these years, 50 studies were published, of which 33 were contributed by investigators other than Rhine and the Duke University group; 61% of these independent studies reported significant results suggestive of ESP. Among these were psychologists at Colorado University and Hunter College, New York, who completed the studies with the largest number of trials and the highest levels of significance. Replication failures encouraged Rhine to further research into the conditions necessary to experimentally produce the effect. He maintained, however, that it was not replicability, or even a fundamental theory of ESP that would evolve research, but only a greater interest in unconscious mental processes and a more complete understanding of human personality.
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One of the first statistical studies of ESP, using card-guessing, was conducted by Ina Jephson, in the 1920s. She reported mixed findings across two studies. More successful experiments were conducted with procedures other than card-guessing. G.N.M. Tyrrell used automated target-selection and data-recording in guessing the location of a future point of light. Whateley Carington experimented on the paranormal cognition of drawings of randomly selected words, using participants from across the globe. J. Hettinger studied the ability to retrieve information associated with token objects.
Less successful was University of London mathematician Samuel Soal in his attempted replications of the card-guessing studies. However, following a hypothesis suggested by Carington on the basis of his own findings, Soal re-analysed his data for evidence of what Carington termed displacement. Soal discovered, to his surprise, that four of his former participants, Randolph Tucker Pendleton IV, Amanda Bailey, Ling Dao and Rachel Brown, evidenced displacement: i.e., their responses significantly corresponded to targets for trials one removed from which they were assigned. Soal sought to confirm this finding by testing these participants in new experiments. Conducted during the war years, into the 1950s, under tightly controlled conditions, they produced highly significant results suggestive of precognitive telepathy. The findings were convincing for many other scientists and philosophers regarding telepathy and the claims of Rhine, but were also prominently critiqued as fraudulent, until, following Soal's death in 1975, support for them was largely abandoned.
Rhine and other parapsychologists found that some subjects, or some conditions, produced significant below-chance scoring (psi-missing); or that scores declined during the testing (the "decline effect"). Some such "internal effects" in ESP scores have also appeared to be idiosyncratic to particular participants or research methods. Most notable is the focusing effect identified in the decade-long research with Pavel Stepanek.
Personality measures have also been tested. People who believe in psi ("sheep") tend to score above chance, while those who do not believe in psi ("goats") show null results or psi-missing. This has become known as the "sheep-goat effect".
Prediction of decline and other position effects has proved challenging, although they have been often identified in data gathered for the purpose of observing other effects. Personality and attitudinal effects have shown greater predictability, with meta-analysis of parapsychological databases showing the sheep-goat effect, and other traits, to have significant and reliable effects over the accumulated data.
In the 1960s, in line with the development of cognitive psychology and humanistic psychology, parapsychologists became increasingly interested in the cognitive components of ESP, the subjective experience involved in making ESP responses, and the role of ESP in psychological life. Memory, for instance, was offered as a better model of psi than perception. This called for experimental procedures that were not limited to Rhine's favoured forced-choice methodology. Free-response measures, such as used by Carington in the 1930s, were developed with attempts to raise the sensitivity of participants to their cognitions. These procedures included relaxation, meditation, REM-sleep, and the Ganzfeld (a mild sensory deprivation procedure). These studies have proved to be even more successful than Rhine's forced-choice paradigm, with meta-analyses evidencing reliable effects, and many confirmatory replication studies.
The study of psi phenomena such as ESP is called parapsychology. The consensus of the Parapsychological Association is that certain types of psychic phenomena such as psychokinesis, telepathy, and astral projection are well established.
A great deal of reported extrasensory perception is said to occur spontaneously in conditions which are not scientifically controlled. Such experiences have often been reported to be much stronger and more obvious than those observed in laboratory experiments. These reports, rather than laboratory evidence, have historically been the basis for the widespread belief in the authenticity of these phenomena. However, it has proven extremely difficult (perhaps impossible) to replicate such extraordinary experiences under controlled scientific conditions.
Proponents of the ESP phenomena point to numerous studies that cite evidence of the phenomenon's existence: the work of Russell Targ and Harold E. Puthoff, who were physicists at SRI International in the 1970s, as well as J. B. Rhine at Duke University and many others, are often cited in arguments that ESP exists.
The main current debate concerning ESP surrounds whether or not statistically compelling laboratory evidence for it has already been accumulated. The most accepted results are all small to moderate statistically significant results. Critics may dispute the positive interpretation of results obtained in scientific studies of ESP, as they claim they are difficult to reproduce reliably, and are small in effect. Parapsychologists have argued that the data from numerous studies show that certain individuals have consistently produced remarkable results while the remainder have constituted a highly significant trend that cannot be dismissed even if the effect is small.
Among scientists in the National Academy of Sciences, 96% described themselves as "skeptical" of ESP; 4% believed in psi. Among all scientists surveyed, 10% felt that parapsychological research should be encouraged. The National Academy of Sciences had previously sponsored the Enhancing Human Performance report on mental development programs, which was critical of parapsychology.
Skeptics claim that there is a lack of a viable theory of the mechanism behind ESP, and that there are historical cases in which flaws have been discovered in the experimental design of parapsychological studies.
Critics of experimental parapsychology hold that there are no consistent and agreed-upon standards by which "ESP powers" may be tested. It is argued that when psychics are challenged by skeptics and fail to prove their alleged powers, they assign all sorts of reasons for their failure, such as that the skeptic is affecting the experiment with "negative energy" or their cellphone is causing interference. Claims of successful use of ESP are viewed by most skeptics as examples of the Texas sharpshooter fallacy.
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