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Following its publication I received some comments about my article about Contact Reflex Analysis. Much of the commentary took place in newsgroups and was the usual drivel about how I didn't know what I was talking about and how it is a wonderful technique, far exceeding the capabilities of such useless real medical practices such as MRI and microbiology. One person just quoted some more slabs from the lying CRA web site, including a claim that arm pushing could somehow be used to cure blindness! The claimant professes to be a Christian, but was unresponsive to my question about whether Jesus used CRA in the healing described in Mark 10:49-52.
One sensible correspondent, however, raised the very real question about how these obvious tricks seem to have some diagnostic power even though the method is transparently ridiculous. The answer is our old friend, cold reading. Consider the iris chart shown at right (click on the image for a larger view). Of all the forms of quack diagnosis, iridology is the one which should be able to be done with a highest level of reliability. The charts are supposed to show a mapping of the parts of the iris to parts of the body. An iridologist should be able to produce a diagnosis using nothing except a picture of someone's eye, without any further examination or feedback from the patient. Using the same chart, another iridologist with the same training should produce a similar diagnosis. This is what happens, for example, with X-rays or other diagnostic scans, where radiologists might differ slightly in how they interpret images but there will be a large area of agreement. It should also be remembered that the radiologist will usually only work from the image, not the physical patient. I know someone who works in a laboratory where Pap smears are examined for signs of cervical cancer. The technicians only have the slides to work with, never the patients, and the whole system only works because a useful level of interoperator reliability can be assured.
I have seen iridologists in action, and they don't seem to refer to the charts at all, although there will always be one on the wall. The iridologist sits opposite the patient and chats about family medical history and other forms of polite small talk. This is how they find the information which is fed back to the patient as a diagnosis. It is the same technique used in mentalist magic acts, but most magicians will admit that it is a trick. (Scamsters like John Edward pretend that it is some secret power, but it is not.) It is, in fact, the basis of most forms of fortune telling where the reader has contact with the subject. In fairness, it must be said that not all readers do it deliberately, because it is quite easy to convince yourself that you have special skills when all you are is an experienced, attentive and sensitive listener.
While it is possible to do iridology from a picture, thus eliminating the possibility of cold reading, this can't be done with something which requires the tester to make subjective measurements of someone else's strength. Again, the deception may not be deliberate and may be driven by unconscious skills and knowledge possessed by the tester (this is how successful diviners work - they unconsciously recognise the likely places to find water). If the tester has to touch the subject and ask them questions about matters which can only be known by the subject then the possibility of cold reading cannot be eliminated. The subject provides feedback which is used by the tester, and many studies have shown that people simply do not remember the details of conversations. When recordings of "successful" readings or diagnoses are compared to subjects' memories of the events, it is almost invariably shown that they overestimate the accuracy of the reader and fail to remember the guesses which missed.
A good tarot reader can tell you about your children and the colour of your car, and a good arm pushing diagnostician can tell you about your diabetes or diverticulitis. What neither can tell you is anything that you do not already know yourself. Their real skill is getting you to tell them what they need to know without you noticing that the telling is happening.