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I've read somewhere long ago that the following suggestibility experiment was performed:

The experimenter asks the subject to hold their palm open, then tells them:

"I'm going to put a very hot coin in your palm, and I want you to hold it for as long as you can".

Then the experimenter places a regular (room temperature) coin in the subject's palm. The subject, visibly distressed, holds the coin for a few seconds, then drops it. In their palm, blisters can be seen.

Here are some similar claims:

  • I think the following question is related, although it's different in the particulars: Did a nocebo study find that rashes could swap hands? – sumelic Nov 13 '16 at 4:25
  • It would be good to see some footage to make sure no-one is falling for the key-blister trick. – Oddthinking Nov 13 '16 at 9:55
  • Now that links to claims have been added, can we please undo the downvotes? If not, should I post a new question? – Dan Dascalescu Nov 13 '16 at 22:12
  • Of course not. That's how you tell if an evil spirit did it vs. suggestion. Right? – PoloHoleSet Apr 17 '18 at 17:47
5

Bottom line: This is an urban legend based on some bad (and long outdated) science.

These types of claims come in many forms, using a coin, ice cube, pen, marker, file, and so on, to purportedly cause blistering or second-degree burns on subjects who believe that they are being touched by some implement (hot coal, shell fragment, branding iron, etc).

A common theme of these stories is that they are a recounting of "legitimate" experiments by "legitimate" researchers. Another common theme is the use of suggestion or hypnosis.

Hypnosis is pseudoscience, so this claim is extraneous. Published research is not usually difficult to find, so it ought to be easy enough to confirm such claims, and indeed, a fair amount of published case reports (mostly before 1950), studies, and reviews on such phenomenon exists in the literature.

One such review, by Orne & Hammer (1974), in the 15th edition of Encyclopaedia Britannica summarizes:

There is controversy about the hypnotic induction of blisters. Told that a cool coin touching him is red hot, a subject shows intense discomfort; sometimes there may be redness and blistering skin. Generally not reproducible under controlled laboratory conditions with normal volunteer subjects, psychologically induced blisters seem most likely to appear only in those prone to convert emotional disturbances into symptoms of skin disease.

Fromm & Shor (1972) (reprinted in Fromm & Shor (2006)) provide a more thorough review in their book on Hypnosis:

During the past one hundred years, many researchers suggested to their hypnotic trance subjects that a blister would form on the skin. With very few exceptions, no skin changes whatsoever were observed (Barber, 1969b, Ch. 9). About twelve researchers reported that suggestions for blister formation gave rise to cutaneous alterations; some of these alterations were labeled "blisters". These reports should be viewed within a broader context ... delineated in a series of reviews (Barber, 1961b; Gorton, 1949; Pattie, 1941; Paul, 1963; Sarbin, 1956; Weitzenhogger, 1953). With very few exceptions, the positive findings were obtained ... prior to the advent of rigorous experimental controls ... A study ... (Borelli, 1953) showed conclusively that the skin change that was produced by suggestion was dermographism, not a blister.

A more recent review by Nash & Barnier (2008), in their textbook on Hypnosis, comes to the same conclusion:

The formation of blisters has been reviewed by others (Gorton, 1957; Paul, 1963; Johnson and Barber, 1976; Barber, 1978; Johnson, 1989; Gauld, 1990), though there have been very few additional studies conducted since Pattie's [1941] review. Paul (1963) suggested that one of the major limitations of a number of prior studies was the failure to consider the possibility of blister formation resulting from contact dermatitis. ... In summary, the notion that hypnotic suggestion can produce blisters does not appear to be reliably validated by controlled research.

Dermographism and contact dermatitis are allergic reactions to contact with materials. In other words, contact with the items listed above can, in some percentage of the population, cause skin reactions that may superficially resemble blisters.

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