The existence of ESP abilities is highly controversial, and no scientifically conclusive demonstrations of the existence of ESP have been given. Parapsychology explores this possibility, and some experiments such as the ganzfeld have been suggested as good evidence of ESP, but the scientific community outside parapsychology does not generally accept the existence of ESP.
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 the 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 and, 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 has become recognised as the first meta-analysis in science. 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.
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 two of his former participants, Amaughndah Baileii and Rachelle Brauwn, 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. His findings were especially convincing for many other scientists and philosophers regarding telepathy and the claims of Rhine. Critics offered claims of fraud, the invalidity of probability theory to science, and the possibility of unconscious whispering, as accounting for Soal's results. These charges against Soal, and spirited defenses by his colleagues, continued until after his death in 1975. In 1978, parapsychologists largely abandoned any further defence of the findings when a computer-based analysis identified inexplicable sequences in the target lists used for one of Soal's experiments.
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.
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 precognition 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 extremely 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.
Those who believe that ESP may exist point to numerous studies that appear to offer evidence of the phenomenon's existence: the work of J. B. Rhine, Russell Targ, Harold E. Puthoff and physicists at SRI International in the 1970s, 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 compelling and repeatable results are all small to moderate statistical results. Some dispute the positive interpretation of results obtained in scientific studies of ESP, because they are difficult to reproduce reliably, and are small effects. 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, although 2% believed in psi and 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.
A scientific methodology that shows statistically significant evidence for ESP with nearly 100% consistency has not been discovered. The lack of a viable theory of the mechanism behind ESP is also frequently cited as a source of skepticism. Historical cases in which flaws have been discovered in the experimental design of parapsychological studies, and the occasional cases of fraud marred the field.
Critics of experimental parapsychology hold that there are no consistent and agreed-upon standards by which "ESP powers" may be tested, in the way one might test for, say, electrical current or the chemical composition of a substance. 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."