Post-hypnotic amnesia occurs when a hypnosis subject is unable to recall earlier events as a result of hypnosis. Usually these events are restricted to those that occurred within the hypnosis session itself. The memories themselves are still intact.
Post-hypnotic amnesia can be induced intentionally by the hypnotist making a suggestion during the hypnosis session. It can also be induced by the hypnotist utilizing the Zeigarnik Effect to close off the subject’s memory of events inside the hypnosis session.
The main reason for inducing post-hypnotic amnesia is to prevent the hypnosis subject from undoing the changes created during the hypnosis session before they can set into long-term memory.
Post-hypnotic amnesia can be permanent, although it is far more common for it to be temporary.
Reasons to use post-hypnotic amnesia
Quite often, hypnosis is used for purposes where recalling the details of the session would inhibit the very reason for the session. This happens in hypnotherapy, where the subject may attempt to undo the changework if they are aware of what was done. And it happens in stage hypnosis, where the entertainment may depend on the subject not knowing.
Post-hypnotic amnesia in hypnotherapy
One of the biggest problems encountered by hypnotherapists is that the subject will attempt to unravel the change. There are numerous reasons for this, but they all have the same result: If the subject is successful, they can neutralize the effects of the hypnotherapy.
To counter this, hypnotherapists will sometimes give suggestions that the subject will not recall any details of the session until some time afterwards. This gives the changes a chance to take root in the subject’s mind, resulting in a higher chance of success.
There is also a school of thought that hypnotherapists should never attempt to block someone’s memory of a session.
There are two competing things going on here. First, as with every other profession, hypnotherapists exist on a sliding scale of skill. There are some hypnotherapists who can get change without inducing post-hypnotic amnesia. There are others who can’t.
Not only that, but many hypnotherapists will encode the change inside covert hypnosis. When we do this, less amnesia is required since the subject generally won’t know what caused the change anyway.
And second, regardless of everything else, it is known that memories remain fluid for a few hours after they’re created or accessed. When we induce post-hypnotic amnesia, it genuinely does give the changes (which are essentially new memories) a chance to properly embed themselves in the subject’s mind.
Ultimately, what it comes down to is that the subject has paid for a change to be made. If the hypnotherapist can do that without inducing amnesia, that’s awesome. But if they can’t, and they do get the results when they induce amnesia, the customer still gets what they paid for.
Post-hypnotic amnesia in stage hypnosis
When hypnosis is for entertainment purposes, it’s often very useful to induce post-hypnotic amnesia. For example, if our act includes a skit where the subject forgets the number 7 and then counts their fingers, it’s not going to work very well unless we actually cause them to forget the number 7.
Similarly, if we want our subject to forget that they’re human and act like an alien, we tend to get much better results if they are able to fully let go and become that alien for a time.
There are countless other examples. Check out any stage hypnosis show for more.
The type of post-hypnotic amnesia used in stage shows typically wears off by the end of the show.
Types of post-hypnotic amnesia
Post-hypnotic amnesia generally fits into 3 broad categories: Overt, covert, and spontaneous.
Overt post-hypnotic amnesia
Overt post-hypnotic amnesia occurs when the hypnotist gives their subject a specific instruction to not recall specific things.
This is usually set up with a post-hypnotic suggestion such as: You will find that any time you attempt to recall any details of this session before tomorrow, your mind will simply jump forward to the moment you awaken from trance, and you’ll feel excited and happy, knowing that there’s something amazing there just waiting to pop out when the time is right.
As with other suggestions, it’s best to riff around the basic ideas, specifically tell them what they will do and what will happen when they try to recall, and specifically tell them that they won’t try to remember.
Because overt post-hypnotic amnesia involves giving the subject direct instructions, it’s possible to use the same process to inhibit their recall of any events from their life, not just those within the session.
Covert post-hypnotic amnesia
Covert post-hypnotic amnesia occurs when the hypnotist intentionally exploits features of the human mind to cause the subject to be unable to remember. This requires more skill than overt post-hypnotic amnesia, but has the advantage that the subject is unlikely to be able to interfere with the suggestion.
Examples of this include telling layered stories, sending subjects on journeys through multiple environments, and even taking steps to induce hypnosis inside the hypnosis.
If you’d like to know more about how this works, you might enjoy my article on How Human Memory Works With Hypnosis.
Spontaneous post-hypnotic amnesia
Sometimes subjects will emerge from hypnosis having no idea what has just happened, despite the hypnotist taking no steps for this to occur.
While this may be caused by unintentional covert post-hypnotic amnesia, it sometimes occurs spontaneously. In these cases, post-hypnotic amnesia is often caused by the beliefs the subject has around hypnosis. Specifically, if the subject believes that people don’t remember what happens to them inside hypnosis, they can find that it’s impossible for them to remember.
Spontaneous post-hypnotic amnesia almost always manifests as the subject being unable to recall anything that happened within the session.