For example, where the subject in an ESP experiment receives a visual cue—the reflection of a Zener card in the holder's glasses—sensory leakage can be said to have occurred.
In the 20th century, J. B. Rhine carried out experiments into ESP. His experiments were discredited due to the discovery that sensory leakage or cheating could account for all his results such as the subject being able to read the symbols from the back of the cards and being able to see and hear the experimenter to note subtle clues.Jonathan C. Smith. (2009). Pseudoscience and Extraordinary Claims of the Paranormal: A Critical Thinker's Toolkit. Wiley-Blackwell. . "Today, researchers discount the first decade of Rhine's work with Zener cards. Stimulus leakage or cheating could account for all his findings. Slight indentations on the backs of cards revealed the symbols embossed on card faces. Subjects could see and hear the experimenter, and note subtle but revealing facial expressions or changes in breathing." Terence Hines has written:
The methods the Rhines used to prevent subjects from gaining hints and clues as to the design on the cards were far from adequate. In many experiments, the cards were displayed face up, but hidden behind a small wooden shield. Several ways of obtaining information about the design on the card remain even in the presence of the shield. For instance, the subject may be able sometimes to see the design on the face-up card reflected in the agent's glasses. Even if the agent isn't wearing glasses it is possible to see the reflection in his cornea.Terence Hines. (2003). Pseudoscience and the Paranormal. Prometheus Books. pp. 119-120
Once Rhine took precautions in response to criticisms of his methods, he was unable to find any high-scoring subjects.Milbourne Christopher. (1970). ESP, Seers & Psychics. Thomas Y. Crowell Company. p. 28 Due to the methodological problems, parapsychologists no longer utilize card-guessing studies.James Alcock. (2011). Back from the Future: Parapsychology and the Bem Affair. Skeptical Inquirer. "Despite Rhine's confidence that he had established the reality of extrasensory perception, he had not done so. Methodological problems with his experiments eventually came to light, and as a result parapsychologists no longer run card-guessing studies and rarely even refer to Rhine's work." Rhine's experiments into psychokinesis (PK) were also criticized. John Sladek wrote:
His research used dice, with subjects 'willing' them to fall a certain way. Not only can dice be drilled, shaved, falsely numbered and manipulated, but even straight dice often show bias in the long run. Casinos for this reason retire dice often, but at Duke, subjects continued to try for the same effect on the same dice over long experimental runs. Not surprisingly, PK appeared at Duke and nowhere else.John Sladek. (1974). The New Apocrypha: A Guide to Strange Sciences and Occult Beliefs. Panther. pp. 172-174
The Turner-Owenby long-distance telepathy experiment was discovered to contain flaws. Frances May Turner positioned herself in the Duke Parapsychology Laboratory whilst Sarah Owenby claimed to receive transmissions 250 miles away. For the experiment, Turner would think of a symbol and write it down whilst Owenby would write her guesses. The scores were highly successful and both records were supposed to be sent to J. B. Rhine, however, Owenby sent them to Turner. Critics pointed out this invalidated the results as she could have simply written her own record to agree with the other. When the experiment was repeated and the records were sent to Rhine the scores dropped to average.
A famous ESP experiment at Duke University was performed by Lucien Warner and Mildred Raible. The subject was locked in a room with a switch controlling a signal light elsewhere, which he could signal to guess the card. Ten runs with ESP packs of cards were used and he achieved 93 hits (43 more than chance). Weaknesses with the experiment were later discovered. The duration of the light signal could be varied so that the subject could call for specific symbols and certain symbols in the experiment came up far more often than others which indicated either poor shuffling or card manipulation.
Possibilities of sensory leakage in the Ganzfeld experiments included the receivers hearing what was going on in the sender's room next door as the rooms were not soundproof and the sender's fingerprints to be visible on the target object for the receiver to see.Richard Wiseman, Matthew Smith, Diana Kornbrot. (1996). Assessing possible sender-to-experimenter acoustic leakage in the PRL autoganzfeld. Journal of Parapsychology. Volume 60: 97-128.Robert Todd Carroll. (2014). "Ganzfeld" in The Skeptic's Dictionary.
Hyman also reviewed the auto-Ganzfeld experiments and discovered a pattern in the data that implied a visual cue may have taken place:
Hyman wrote the auto-Ganzfeld experiments were flawed because they did not preclude the possibility of sensory leakage.
Thomas Gilovich has written:
According to Marks, when the cues were eliminated the results fell to a chance level.David Marks, Richard Kammann. (1980). The Psychology of the Psychic. Prometheus Books. Marks was able to achieve 100 per cent accuracy without visiting any of the sites himself but by using cues.Martin Bridgstock. (2009). Beyond Belief: Skepticism, Science and the Paranormal. Cambridge University Press. p. 106. "The explanation used by Marks and Kammann clearly involves the use of Occam's razor. Marks and Kammann argued that the 'cues' - clues to the order in which sites had been visited - provided sufficient information for the results, without any recourse to extrasensory perception. Indeed Marks himself was able to achieve 100 per cent accuracy in allocating some transcripts to sites without visiting any of the sites himself, purely on the ground basis of the cues. From Occam's razor, it follows that if a straightforward natural explanation exists, there is no need for the spectacular paranormal explanation: Targ and Puthoff's claims are not justified." James Randi has written controlled tests by several other researchers, eliminating several sources of cuing and extraneous evidence present in the original tests, produced negative results. Students were also able to solve Puthoff and Targ's locations from the clues that had inadvertently been included in the transcripts.James Randi. (1997). " Remote viewing" in An Encyclopedia of Claims, Frauds, and Hoaxes of the Occult and Supernatural. St. Martin's Griffin.
Marks and Kamman concluded: "Until remote viewing can be confirmed in conditions which prevent sensory cueing the conclusions of Targ and Puthoff remain an unsubstantiated hypothesis."C. E. M. Hansel. (1980). ESP and Parapsychology: A Critical Reevaluation. Prometheus Books. p. 293
In 1980, Charles Tart claimed that a rejudging of the transcripts from one of Targ and Puthoff's experiments revealed an above-chance result.Charles Tart, Harold Puthoff, Russell Targ. (1980). Information Transmission in Remote Viewing Experiments. Nature 284: 191. Targ and Puthoff again refused to provide copies of the transcripts and it was not until July 1985 that they were made available for study when it was discovered they still contained sensory cues.Terence Hines. (2003). Pseudoscience and the Paranormal. Prometheus Books. p. 136 Marks and Christopher Scott (1986) wrote "considering the importance for the remote viewing hypothesis of adequate cue removal, Tart's failure to perform this basic task seems beyond comprehension. As previously concluded, remote viewing has not been demonstrated in the experiments conducted by Puthoff and Targ, only the repeated failure of the investigators to remove sensory cues."David Marks, Christopher Scott. (1986). Remote Viewing Exposed. Nature 319: 444.
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