A post-hypnotic suggestion is, as one might expect from the name, a suggestion given to a person in a hypnotic state that later influences the behavior of said person once he or she is no longer in a hypnotic state. A post-hypnotic suggestion is not, however, a command, in the sense that it can be given with the intention of forcing a hypnotic subject to behave in a manner contradictory with his or her immediate intended behavior: Post-hypnotic suggestions, except potentially in cases of extreme brainwashing, are simply not powerful enough to contradict the willful, conscious behavior of the subject.

Hypnosis and post-hypnotic suggestions go together very often, but they are not one in the same, and hypnosis can exist and be useful without leaving any suggestions behind. Self hypnosis can be used as a particular form of meditation, for instance, just to relax or to focus.

I am not a hypnotist, or trained in hypnotherapy in any sense, but I've been interested in suggestivity and hypnosis, along with the more scientific neuro-linguistic programming for a number of years. For what it's worth, I've never met with much success with self hypnosis, and have never given myself a post-hypnotic suggestion. I haven't tried very hard.

What can post-hypnotic suggestions do?

Post-hypnotic suggestions, are used to make behavior modifications on a subconscious level. Richard Bandler and John Grinder, in some of their older NLP books, describe dramatic post-hypnotic behavior, ranging from transposing sensory behavior (Such as cigarettes tasting like dirt instead of cigarettes or touches being felt in different parts of the body instead of the part touched) to one particular example of a woman who could look at someone and have the text of that person's name appear in her vision so that she could recognize them, even years after a single meeting. A more common, and certainly less labor-intensive to instill, use would be to form associations between concepts, such as leaving a feeling of displeasure whenever cigarettes or fattening of food are thought of, in an effort to dissuade the subject from smoking or eating those foods.

As part of a stage hypnosis act, a post-hypnotic suggestion, assuming that hypnosis has actually taken place, can be used leverage the expectations and acquiescence of the subject to perform the various antics that stage hypnosis shows are known for. I have seen it argued, however, that in a large number of cases the subjects in stage hypnosis acts achieve very little trance at all, and instead are mostly influenced by their expections of how they will be "forced" to act, and what the audience and dominating personality of the stage hypnotist expect of them. Certainly any sort of trance could be used to reinforce these feelings of expectation. The feasability of inducing a profound hypnotic state quickly and in public is a matter for discussion elsewhere.

What can't post-hypnotic suggestions do?

Post-hypnotic suggestions have less influence over a subject's behavior than suggestions that are experienced completely in an entranced state. In the above example of stage hypnosis, for instance, it may be reasonable to assume that a post-hypnotic suggestions, properly instilled, could cause someone to feel the urge to cluck like a chicken on hearing a bell, but it could not produce the profound physical feats such as the infamous "person across two chair backs" skit.

It would be, in my (somewhat uninformed) opinion, quite difficult to set up a post-hypnotic suggestion that the subject would not realize is occuring on some conscious level. It may be possible to do so given prolonged work with a hypnotist. Along those same lines, a post-hypnotic suggestion could, in any reasonable circumstance, be consciously shrugged off unless it affected an area of experience that humans have absolutely no conscious control over.

Post-hypnotic suggestions that go against the desires of the subject have very little, of any, effect on him or her. One of the reasons that the suggestions work is because, on some level, the subject wants them to, and has been convinced that following the suggestion is the best way to proceed. A lot of hypnotic technique, as far as my understanding goes, involves getting the subject to believe this profoundly enough to have the suggestion "stick."

How does one give a post-hypnotic suggestion to someone under hypnosis?

The general idea behind instilling a post-hypnotic suggestion in a hypnotized subject is that the subject must be persuaded to follow the suggestion once the hypnotic state is done with. This requires some careful planning unless the subject really wants to follow the suggestion. Word choice, tone of voice, the quality of trance, and repetition are all important to making it work.

Word choice, in particular, seems to be a very big subject with NLP practitioners like Bandler and Grinder. The general idea is that the suggestion must be reasonable, believable, and desirable. A suggestion that sounds like it's coming totally out of left field probably won't get interpreted the way you want it to. Precision is necessary, but too much precision will bog down the subconscious in too many details. Telling someone that they're Superman won't work so well (Except if they want to go along with it) because the mind knows that Superman is a fictional character. Likewise, telling a person they can fly won't work so well: The subconscious is well aware of the physical limits of reality.

Desirability can be a tricky thing. It cannot be readily assumed from conscious thought: A person who says they want to stop smoking may not really want to stop smoking, even if they think they do. If the subject really does desire to undertake the action in the suggestion, then the hypnotist's job is considerably easier. If not, then the way to go about things is to reframe the problem in such a way as to convince the subconscious that following the suggestion is the best way to go. The line of reasoning may go like this:

1. Cigarettes provide a nice high but they're bad for your health.
2. The health risks outweigh the nice feelings.
3. It would be too hard to stop smoking cold, but by making cigarettes taste like dirt, they'll be less desirable and the conscious mind won't want to smoke them anymore.
4. Therefore, making cigarettes taste like dirt is the best way to stop smoking.

I make no claim that this is the best way to phrase this -- I'm shooting from the hip on examples. Note, however, that I specifically don't address the alternatives. A person in a hypnotic state tends to be rather gullible, so giving a list of truths then making a small leap of logic often works fine.

After making the case, all that's left for the hypnotist to do is to repeat it enough times to become engrained in memory and remembered, even (especially) once the conscious mind isn't paying attention to it anymore. Repetition is a fundamental aspect of hypnosis, and isn't very important for this node to hit, so I'll leave it for elsewhere.

How effective are post-hypnotic suggestions?

Post-hypnotic suggestions of any significant weight require a lot of time and effort to instill. They fade quickly over time, and require considerable reinforcement and upkeep in order to really stick for any significant length of time. Even then, very strong suggestions will fade over a period of weeks or months. Even if you're convinced that cigarettes are _made_ of dirt, they'll eventually just be cigarettes to you again at some point in the future unless other actions happen to continuously reinforce the suggestion. (Hopefully by then you've quit, eh?). It's been shown that even brainwashing techniques fade over the span of years, and no hypnotist you're ever going to meet can match the CIA or the KGB for brainwashing power!

As far as the malleability of behavior under the influence of a suggestion, I wouldn't hope for too much without years of reinforcement. The example Bandler and Grinder give about the woman who has names appear before her eyes must have taken a tremendous effort of self-hypnosis in order to achieve, and even that is a fairly modest physical outcome -- It's all mental, which is where hypnosis really has its strengths. The old Flintstones (I think it was) skit of acting like a chicken when you hear a bell ring after just one time with a hypnotist just can't happen. Likewise, as much as we all like to fantasize, Office Space's treatment of the subject isn't very realistic, either.

If you're interested in learning to stop smoking, or perhaps just in playing some games with your mind, however, you may well meet with good success with hypnosis involving post-hypnotic suggestions. They are known to be an effective means of reinforcing conscious effort to alter behavior. Though plenty of people are more than happy to assign this to the placebo effect, in some sense, that's all that a post-hypnotic suggestion really is anyway, just phrased in such a way as to be very believable. Your milage may vary, but if you're going to try to change behavior using NLP or hypnosis and it doesn't work, at least you'll get some nice relaxation out of it.

References

I didn't use any specific references while making this node, because good, objective information on hypnosis is hard to find online. (See speed seduction for a good example of other stuff you can find.) I recommend reading the aforementioned work of Richard Bandler and John Grinder for more information on NLP, and visiting http://www.hypnosis.com/ for information on general hypnosis. Milton H. Erickson started the whole NLP idea, so his books are also likely to be of use on the subject. I recommend a trip to the public library.

Comments and corrections are welcome. Just /msg me. I'll update as needed. Someone more informed than me, speak up!