I read an interesting article on New Scientist today describing how new research is suggesting that those who are un-hypnotisable may have a more balanced brain (between left and right hemispheres) than those that are highly susceptible!
There is a kind of standard statistic in hypnosis that (whether through suggestion or genuine experience) most hypnotists will agree on:
About 20% of the population is highly susceptible
About 60% of the population can be hypnotised
About 20% of the population cannot be hypnotised
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