Sensory leakage is a term used to refer to information that transferred to a person by conventional means (other than psi) during an experiment into ESP.
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.
History
Scientists such as Donovan Rawcliffe (1952), C. E. M. Hansel (1980), Ray Hyman (1989) and Andrew Neher (2011) have studied the history of psi experiments from the late 19th century up until the 1980s. In every experiment investigated, flaws and weaknesses were discovered so the possibility of naturalistic explanations (such as sensory cues) or deception and trickery were not ruled out. The data from the Creery sisters and the Soal–Goldney experiments were proven to be fraudulent, one of the subjects from the Smith-Blackburn experiments confessed to fraud, the Brugmans' experiment, the experiments by John E. Coover, Joseph Gaither Pratt, and Helmut Schmidt did not rule out the possibility of sensory cues or trickery.
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. 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.
Once Rhine took precautions in response to criticisms of his methods, he was unable to find any high-scoring subjects. Due to the methodological problems, parapsychologists no longer utilize card-guessing studies. 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.
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.