Detecting Deception

When the suspect is the source of the information, additional factors are also important beyond those of memory retrieval. These often relate to the need to determine if a person is attempting to deceive the interviewer. Thus although there are many objective, conventional police strategies for detecting deception, most obviously determining if the known facts contradict the suspect's claims, there are a number of situations in which some knowledge of behavioural and psycholinguistic cues to deception would be very helpful. A number of researchers, most notably Ekman (e.g. Ekman and O'Sullivan, 1991), have claimed that such cues are available, but others are more sceptical as to the possibility of any generally available indexes of deception from the actions or words of the suspect during a police interview (Edelman, 1999).

There is much more evidence to indicate that for many people there are psychophys-iological responses that may be indicators of false statements (e.g. Kleiner, 1999). The procedure for examining these responses is often referred to as a polygraph or 'lie detector'. In essence this procedure records changes in the autonomic arousal system, i.e. emotional response. Such responses occur whenever a person perceives an emotionally significant stimulus. The most well-established indicator is when the respondent is asked to consider information that only the perpetrator would be aware of, known as the 'guilty knowledge' test.

A more controversial procedure is to ask 'control questions' that many people would find emotionally significant in order to determine if they elicit responses that can be distinguished from those questions relating directly to the crime. However, in both these applications of psychophysiological measures the most important element is the very careful interview procedure before measurements are made and during the process. In general the technique is more productive in supporting a claim of innocence than in providing proof of guilt. For this reason many jurisdictions do not allow 'lie detector' results to be presented as evidence in court.

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