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Modification of suggestibility

A key question in hypnosis research is whether or not suggestibility is modifiable. On the one hand there is evidence that hypnotic suggestibility is a stable personality trait - individuals have been shown to score similarly over periods of up to 25 years (Hilgard, 1965; Piccione et al, 1989). This is backed up by data showing that the similarity of hypnotizability of identical twins is higher than the similarity of hypnotizability of non-identical twins (Morgan, 1973) (So called 'twin data' like this is often used by scientists to determine whether there is a genetic component to whatever trait they are measuring).

However, suggestibility can also be measured in the absence of hypnosis - just administer the test suggestions without administering the hypnotic induction. In the past this has been known as 'primary suggestibility', or 'non-hypnotic suggestibility', and a more recent label is 'imaginative suggestibility' because participants are often asked to imagine the suggested state of affairs (Kirsch & Braffman, 2001).

Mofication of suggestibility with hypnosis

Since suggestibility can be measured before and after a hypnotic induction, one of the most obvious tests is to see whether such an induction increases suggestibility. Kirsch and Braffman (2001) report that this has been done is six studies and that the results are remarkably consistent, revealing that the effects of hypnosis are fairly small - in the region of 1 to 2 points on a standard 12-point scale. Kirsch and Braffman (2001) term this difference between hypnotic and non-hypnotic suggestibility as 'hypnotizability', and note that the correlation between an individual's scores is very high.

(Some researchers have pointed out that people might slip into 'trance' spontaneously, even if a hypnotic induction is not administered, and argue that responses to suggestion should be taken as evidence that the person is in a hypnotic state. However this argument is circular. People are argued to be responding to the suggestions because they are in a trance, but the only reason to think they are in a trance is because they are responding to suggestion).

Modification via other psychological approaches

Many researchers have investigated using other psychological techniques to increase suggestibility. One of the most famous of these methods is the cognitive-behaviourally oriented Carleton Skills Training Programme (CTSP: Gorassini & Spanos, 1986). The CTSP is a 75 minute procedure which is designed to increase motivation and to teach direct management strategies (insisting that the participant make suggested responses and not just wait for them to happen, and instructions to become more absorbed). The CTSP has been studied in at least ten different laboratories and it is reported that it is common for at least 50% of participants who initially score low prior ot training score high afterwards. Some researchers have argued that increases brought about through this method are not genuine and are simply the result of participants being encouraged to respond without necessary experiencing their response as involuntary (a criteria Weitzenhoffer set as a 'true' response to suggestion). However, other researchers have argued that the subjective responses of such 'trained high suggestible' participants are indistinguishable from participants who were 'naturally high suggestible' without training. They also argue that training generalizes to new, untrained, suggestions, and that the effects persist for up to a 4-month follow-up (Gorassini, 2004).

Another interesting approach to suggestibility modification has been research into the label 'hypnosis'. Gandhi & Oakley (2005) administered a set of eight standard test suggestions before and after the administration of a hypnotic induction. The twist was that half of the participants were told that the induction was hypnotic, whereas of the other half all mention of hypnosis was removed and replaced with the word 'relaxation'. They found that the 'relaxation' induction produced a modest increase in suggestibility, but that the increase in suggestibility was very significant if labelled hypnosis. These data indicate that an individual's perceptions and expectations regarding a hypnotic procedure can have dramatic effects upon their the procedure's effectiveness.

Modification of suggestibility with drugs

A number of researchers have tried administering drugs to increase suggestibility. Typically the methodology of these studies involves either testing the suggestibility of two groups of participants while administering one group the drug and the other group the placebo. Large numbers of participants are necessary in this design to avoid chance variations in suggestibility affecting the results. An alternative method is to test the same person twice, once before and once after taking the drug. In many ways this is a more reliable method, although it may introduce biases because of test-retest effects - participants becoming familiar with the tests. The results of the more reliable studies are presented below.

Study

Drug

Suggestibility Scale

Suggestibility Change %

Sjoberg et al (1965)

Mescaline

SSS:A/B*

12.7

LSD

SSS:A/B*

12.8

Psilocybin

SSS:A/B*

0.66

Combination

SSS:A/B*

11.75

Gibson et al (1977)

Diazepam

SHSS:A/B**

0.91

Active placebo

SHSS:A/B**

0.41

Kelly et al (1978)

Cannabis

HGSHS***

22.6

Control

HGSHS***

0.54

Barber et al (1979)

Nitrous oxide

Bespoke

36

Nitrous oxide

Bespoke (excl analgesia

27.5

Table 1: Details of previous studies investigating pharmacological manipulation of suggestion. * Stanford Suggestibility Scale (Weitzenhoffer & Sjoberg, 1961). ** Stanford Hypnotic Susceptibility Scale (Weitzenhoffer & Hilgard, 1959). *** Harvard Group Scale of Hypnotic Susceptibility (Shor & Orne, 1962)

Many of the drugs tested have widely varying pharmacological effects and it is now yet known whether suggestibility increases are due to a specific neurochemical action (or actions), or whether general dissociative ('woozy') effects of the drugs lead to suggestibility change, possibly via a psychological mechanism. We need to know more about the neurobiology of response to suggestion (see the section on neuroimaging studies) to answer this question more fully.

The future of suggestibility modification

The results presented above indicate that it is possible to increase responses to suggestions, potentially to quite a substantial degree. Given that some clinical uses of hypnosis and suggestion, such as pain relief, are known to vary in their effectiveness depending upon how susceptible patients are it could be enormously valuable to be able to modify suggestibility. Some pioneering research has already indicated that hypnosis can be beneficial alongside conscious sedation (Faymonville et al, 1997).

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