Podium: How hypnosis plays tricks on police witnesses

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From a speech by the lecturer in psychology at Liverpool University to the British Association Festival of Science

AN EYEWITNESS who says, "That is the man who attacked me! I'm absolutely sure of it!" provides convincing evidence. Consequently it is not surprising that eyewitnesses are a crucial part of many police investigations, although they rarely remember as much as the police would like. Unfortunately, eyewitnesses are also usually implicated in false convictions.

This is illustrated by the recent case of Kenneth Adams, who was convicted of two counts of murder and one of rape, based on witness identification. He served 16 years in prison before being exonerated by DNA evidence.

Because of the role of eyewitness evidence in the criminal justice system, forensic psychologists have devoted a great deal of time and energy to devising ways of improving eyewitness performance. One method that seems to have the potential to enhance witness memory is hypnosis, and in real police investigations it has apparently met with considerable success. Furthermore, most members of the public and practitioners of hypnosis believe it can improve memory, and many hypnotists are willing to offer their services to the police.

However, laboratory studies have shown three main problems with hypnosis. First, increases in correct recall are often associated with very great increases in incorrect recall (ie accuracy declines). Second, confidence often increases regardless of accuracy (eg subject-eyewitnesses are often very confident of the accuracy of incorrect information). Third, subject- eyewitnesses often show increases in suggestibility to leading questions and misleading post-event information (eg subject-witnesses are more likely to incorporate suggested details in their accounts). Clearly, these problems could have an adverse effect on a police investigation.

Ironically, the problems associated with hypnosis contribute to the impression that it is a useful technique. Forensic hypnosis is likely to produce more information, but much of this is likely to be incorrect. Hypnosis is usually used when the police have no other leads, and so checking the information an eyewitness gives is difficult. Therefore, without verification, it creates the appearance of memory enhancement when in fact it is creating memory distortion. In addition, when facts cannot be checked we tend to use witness confidence to determine accuracy. Because confidence typically increases with hypnosis, it appears to produce more accurate testimony.

Although hypnosis has the potential to distort eyewitness memory, other factors associated with hypnosis may have a positive influence on recall. For instance, as well as a formal hypnotic induction procedure, most hypnotic interviews include components of a procedure known as the cognitive interview. Essentially this involves asking the witness to reinstate mental context, report everything, recall events in a different order and change perspectives. Research shows that, without hypnosis, the cognitive interview on its own has the potential to enhance recall.

Over a number of studies, memory distortion is frequently reported with hypnosis (though not always). This is not the case with the cognitive interview, and information elicited with the cognitive interview is unlikely to face legal challenge. Therefore, the cognitive interview is a more suitable technique for forensic interviews.

In summary, although memory distortion is possible without hypnosis, it is more likely with hypnosis. This problem, combined with the legal problems associated with hypnosis, means that it is inadvisable to use hypnosis to try to enhance eyewitness memory. However, an alternative might be to use the cognitive interview. It has the potential to enhance recall without the memory distortions associated with hypnosis.

Most UK police forces now include cognitive interview training as part of officers' training. Interestingly, though, a role may still exist for forensic psychologists to interview eyewitnesses. Research indicates that police officers have difficulty using the cognitive interview. For example, few trained officers use all the components, and most appear unable or unwilling to change their interviewing style to reduce negative behaviours such as interrupting and asking closed questions. It could be the case, therefore, that forensic psychologists may maximise eyewitness recall by using their psychological skills to apply the cognitive interview effectively.