In 1917, Edward Clodd analyzed the mediumship of the trance medium Gladys Osborne Leonard and came to the conclusion that Leonard had known her séance sitters before she had held the séances, and could have easily obtained such information by natural means.[119] The British psychiatrist Charles Arthur Mercier wrote in his book Spiritualism and Sir Oliver Lodge (1917) that Oliver Lodge had been duped into believing mediumship by trickery and his spiritualist views were based on assumptions and not scientific evidence.[120]
In 1991, Wendy Grossman in the New Scientist criticized the parapsychologist Stephen E. Braude for ignoring evidence of fraud in mediumship. According to Grossman "[Braude] accuses sceptics of ignoring the evidence he believes is solid, but himself ignores evidence that does not suit him. If a medium was caught cheating on some occasions, he says, the rest of that medium's phenomena were still genuine." Grossman came to the conclusion that Braude did not do proper research on the subject and should study "the art of conjuring."[183]
In 1910 at a séance in Grenoble, France the apport medium Charles Bailey produced two live birds in the séance room. Bailey was unaware that the dealer he had bought the birds from was present in the séance and he was exposed as a fraud.[114] The psychical researcher Eric Dingwall observed the medium Bert Reese in New York and claimed to have discovered his billet reading tricks.[115] The most detailed account at exposing his tricks (with diagrams) was by the magician Theodore Annemann.[116]

Senses used by mental mediums are sometimes defined differently from in other paranormal fields. A medium is said to have psychic abilities but not all psychics function as mediums. The term clairvoyance, for instance, may include seeing spirit and visions instilled by the spirit world. The Parapsychological Association defines "clairvoyance" as information derived directly from an external physical source.[37]