The only true reason hypnosis would not work is conscious resistance. An argument can be made for subconscious resistance causing hypnosis to fail.
The measure of “effect” is important because the subjects self-report is inevitably unreliable weather he or she confirms or denies feeling hypnotized.
Before going into the “does hypnosis work” question; I will say that when hypnosis doesn’t work it is most likely because it wasn’t done correctly. That being said; the issue of hypnotisability/suggestibility is extensively discussed in hypnosis to describe the degree to which people respond to hypnosis.
When it comes to hypnotisability we call the most easily hypnotisable subjects somnambulists. We also have types of suggestibility including physical, emotional or rational suggestibility. Physical suggestibility is the response to direct commands whereas emotional suggestibility is the response to implied commands and metaphor. They also exhibit different latency periods between suggestion and response with emotionals having longer periods. Rational suggestibility is the response to rationalization; and these types of subject require reasons attached to every suggestion because they are usually more critical and hyper vigilant.
Coming back to hypnotisability; we divide subjects into quartiles and people are roughly distributed equally between
1- not hypnotisable
2- slightly hypnotisable
3- moderately hypnotisable
4- extremely hypnotisable/somnambulists
That being said the category of not hypnotisable most likely indicates not responding to direct command rather than to the whole spectrum of suggestion categories. It is likely using indirect command or rationalization or even a shock induction would work on these classes of subjects.
And so we boil down to measuring effect and how we know hypnosis works. To begin with we have signs of trance that the hypnotist may or may not note. There are various checklists of symptoms but the idea is that the subject looks hypnotized and is compliant. Compliance and engagement in the process is another sign hypnosis is working but may be an invalid measure in techniques like the coma state. Finally, and most importantly is the focus on expected changes. Did the subject accomplish the expected behavioral outcomes? Is the phobia gone? Did he or she exhibit the phenomena sought after in hypnosis?
Hypnosis works with everyone. The exceptions are conscious and subconscious resistance or a lack in communication skills and intelligence. You have to know what you are looking for in able to reliably track it.
