A post hypnotic suggestion refers to a suggestion which is given to a person when in a hypnotic state and which involves an action to be carried out later when the person is in their normal waking state. It can be given in hetero-hypnosis and self-hypnosis. Take the following example, which is simple and of no particular use in itself, except to demonstrate typical, posthypnotic behavior. When in a hypnotic state, suppose you gave yourself a suggestion that when you awoke after five minutes you would have a tremendous desire to take off your right shoe, and that with each moment that passed this urge would become stronger and stronger until you took off your right shoe. If you were actually carrying this out you would use repetition and imagery. Now, in all probability, you would carry out this instruction. After five minutes had elapsed, you would find yourself thinking about your right shoe, your attention would more and more be directed at your right shoe. You will probably remember the posthypnotic suggestion, but still the urge to comply will get stronger and stronger and stronger, until you carried out what had been suggested.
If a post hypnotic suggestion is not carried out, two reasons are usually advanced to explain this. First, the person was not in a sufficiently deep enough trance. Second, the instruction was ridiculous, inconsiderate or improper. Let me take each of these in turn. The first explanation is certainly true in a number of occasions, but it is a fallacy to conclude the depth of hypnosis from the fact of whether or not a given posthypnotic suggestion is carried out. Post hypnotic suggestions, if reasonable, can be acted upon with the person being in a light, medium or very deep trance.
The second explanation has arisen largely from experimentation and stage shows. If you are given a ridiculous instruction, in either hetero-hypnosis or in self-hypnosis, then in all likelihood you will not comply. One may consider the above suggestion to take off your right shoe after five minutes, comes into such a category. If you do, then you may not carry out this particular posthypnotic suggestion. The same is true if the suggestion, in some way, went against your moral code. It appears that the super-ego, to use a Freudian concept, is always working and does not ‘go to sleep’. The point here is that these situations tend to arise in hetero-hypnosis, where a ridiculous suggestion is being used in experimental work to ‘demonstrate’ the existence of posthypnotic phenomenon; or by stage hypnotists who wish to create an amusing performance (at the cost of the volunteer). In self-hypnosis this is unlikely to arise because you are unlikely to give yourself ridiculous or improper posthypnotic suggestions.