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The most potentially fatal flaws in a psi study are those that would allow a receiver to obtain the target information in normal sensory fashion, either inadvertently or through deliberate cheating. This is called the problem of sensory leakage. --Daryl J. Bem
Sensory leakage is a term used to refer to ways information might be transferred by ordinary sensory means during a controlled psi experiment. A controlled psi experiment is supposed to measure transfer of information or energy in ways that are currently inexplicable in terms of known physical or biological mechanisms. Sensory leakage is the transfer of information by non-psychic means. There are various ways this can happen. Participants or experimenters could be unintentionally cuing subjects. Inadequate controls could allow participants to see or hear things by ordinary means.
Since most psi experiments depend on statistical analyses of data—rather than direct observation of a subject, say, picking the winning numbers of the SuperLotto five minutes before the drawing—it is very important that there be a high degree of confidence that what is being measured is not the transfer of information by non-psychic means.
For example, in a Zener card telepathy/clairvoyance experiment a sender may look at a card and try to send information about this card to a receiver by telepathic means. There are various kinds of sensory leakage that should be controlled for. The cards should not be visible to the receiver. If the receiver were able to see through the card held by the sender or were able to see a reflection of the card in a mirror or off a window or the glasses of the sender, information might be transferred.
In a ganzfeld or remote viewing experiment, when the target and controls are shown to the receiver, they should not have been handled by anyone nor should they be dated or marked in any way that might convey information to the subject. Furthermore, the experimenter who presents the receiver with the target and controls should be blind to the actual target, lest he inadvertently give subtle cues to the receiver. As Jeffrey Mishlove says:
Both judges and percipients may detect creases, marks, smudges, temperature differences or other artifacts that result if actual targets have been handled and then mixed in with targets from a pool for judging. Handling cues may also result when targets placed in envelopes are opened and then resealed or placed in new envelopes, as has sometimes been done.
In some experiments it is essential that the rooms of sender and receiver be soundproofed to prevent transfer of information by known physical or biological mechanisms, such as overhearing a video being played or a conversation taking place.
Mishlove notes that:
As early as 1895, psychologists described "unconscious whispering" in the laboratory and were even able to show that senders in telepathy experiments could give auditory cues to their receivers quite unwittingly.... Ingenious use of parabolic sound reflectors made this demonstration possible. Many researchers in the early years of experimental psychology and psi research gave early warnings on the dangers of unintentional cueing....The subtle kinds of cues described by these early workers are just the kind psychologists have come to realize mediate experimenter expectancy effects found in laboratory settings.
Mishlove lists a number of precautions that psi experimenters must take to prevent sensory leakage.
In designing experiments to prevent sensory leakage, experimenters cannot assume that there are no tricksters present among the subjects. Precautions must be taken that would prevent the most skilled of tricksters or magicians from succeeding in obtaining normal sensory information about the targets.
In short, the competent psi experimenter must take every reasonable precaution to prevent information from being transferred inadvertently or intentionally by either experimenters or subjects.