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The decline effect is the notion that psychics lose their powers under continued investigation. This idea is based on the observation that subjects who do significantly better than chance in early trials tend to do worse in later trials. (Parapsychologists refer to the incline effect to describe those subjects who tend to do better in the later trials.)
The terms was coined by J. Beloff, who wrote:
...it soon transpired that a decline effect, for ESP no less than for PK, could persist across sessions and, ultimately, across an entire career. Nearly all the high-scorers eventually lost their ability. Even Pavel Stepanek, whose 10-year career as an ESP subject earned him a mention in the Guinness Book of Records, eventually ran out of steam. When, after a long break, he was retested recently by Dr Kappers in Amsterdam, he could produce only chance scores. I do not think it was loss of motivation or boredom in his case, as has sometimes been put forward as an explanation for the long-term decline effect, for it was Stepanek's great strength that he was constitutionally incapable of ever being bored! Nor can we take seriously Martin Gardner's attempt to explain how he might have relied throughout on trickery. If indeed he was a trickster, he should have steadily improved as he became more practiced. Whatever the explanation of these long-term declines, it must surely be something deep and pervasive.*
Beloff is incorrect in assuming that a trickster should steadily improve as he becomes more practiced. This is likely only if those testing the trickster are incompetent at detecting deception.
An alternative explanation for the decline effect is that as time goes on or as others do the testing, tighter controls and better experimental protocols are used, leading to the poorer scores. For example, transparent Zener cards were used in some of J. B. Rhine's experiments. Observers may have been brought in for later trials, making it more difficult for the subject to cheat.
Another alternative explanation for why subjects who do well in early trials tend to do worse in later trials, and vice-versa, is regression toward the mean. That is, one would expect, over time, that a subject without psychic ability would approach chance levels in guessing experiments but that there would be occasional runs where the subject would do better than chance and runs where the subject would do worse than chance. These deviations would have nothing to do with the waxing and waning of psychic powers but to the very natural tendency of regression toward the mean.
Beloff's explanation for the decline effect was that paranormal phenomena represent a "violation of the natural order" and Nature "reacts to these rents in the fabric of the cosmos by healing them just as our bodies heal wounds."* This is pure speculation on Beloff's part, however.
Others blame boredom, fatigue, or the inability to control the unconscious processes underlying psi.*
The alternative explanations have the merit of not only being more plausible than the decline effect theory, but also of applying to types of psi tests, such as Zener card tests, over time. That is, it is likely that scientific experimenters have simply gotten better at evaluating their data and at designing their experiments to control for such things as sensory leakage or cheating, and that is why scores have declined.
See also clairaudience, clairvoyance, displacement, ESP, experimenter effect, ganzfeld experiment, Uri Geller, medium, Raymond Moody, Nostradamus, optional starting and stopping, parapsychology, past life regression, precognition, psi, psi assumption, psi-missing, psychic, psychic detective, psychic surgery, psychokinesis, psychometry, James Randi psychic challenge, remote viewing, retrocognition, séance, Charles Tart, telekinesis, telepathy, and James Van Praagh.
The ABC of ESP by Arthur KoestlerThe Country of the Blind by Arthur Koestler
Telepathy and Clairvoyance by Raynor C. Johnson