Een leugendetector heeft zo goed als geen waarde. Ze heeft een accuraatheid van vermoedelijk ongeveer 50% en dat betekent dat je evengoed kop of munt kan gooien om te bepalen of een getuigenis waar is of niet. Bovendien meet een leugendetector enkele lichamelijke signalen die gelinkt zijn met stress. Iemand die daar een beetje op oefent, kan evengoed liegen maar lichamelijk geen tekens van stress vertonen.

Psychopaten vertonen sowieso veel minder emoties, dus is het voor hen een koud kunstje om een leugendetector te omzeilen. Omgekeerd, als iemand niet liegt is, maar bv. angstig is, kan die evengoed somatische tekens van stress vertonen en dus, volgens de detector, liegen. Zoals iemand het hier al stelde: je kan evengoed Tarotkaarten gebruiken om te voorspellen of Tinck de waarheid spreekt of niet.

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The name "lie detector" is misleading in many ways. First, the polygraph doesn't actually detect lies but, instead, measures arousal. It is based on the idea that we will be a little more stressed, with fleeting changes in blood pressure, sweat gland activation and respiration, when answering questions with lies compared to giving truthful responses. The majority of tests involve comparing responses to control questions that the interviewee will respond truthfully to ("Are you sitting in a chair?") with responses to investigation-relevant questions ("Did you handle the money?").

The "lie detection" part comes from an interpretation of the differences in arousal between these types of answers. But physiological differences may arise for many reasons, not just from intentional deception – I may become more stressed if I worry that I won't be believed, or if the question concerns something that is naturally arousing – perhaps even just a question that contains highly emotional words.

Because there is no pattern of arousal that is unique to deception, the decision to classify a set of responses as untruthful is inevitably a leap from the shaky ground of ambiguous data into the fog of inference. As a result, techniques to "beat" a polygraph are simple and effective. The simplest strategy seems to be to increase arousal during the control questions, rather than trying to reduce arousal during deception, to eliminate any difference.

Second, if I want to know that someone is telling the truth or not, I want to make sure that the technology produces as few false positives and false negatives as possible. For example, just as I can correctly punish all criminals by throwing everyone in jail (innocents, or false positives, be damned), I could correctly "detect" 100% of lies just by classifying every answer as a falsehood. Accuracy, however, requires a balance.

Reviews of the scientific evidence by the National Research Council in the US and the British Psychological Society in the UK have indicated that the polygraph has an accuracy of about 85% when evaluating genuinely guilty people. Unfortunately, the accuracy is probably nearer to 50% (with results here varying greatly across studies) when attempting to do the same with genuinely innocent people.

Bron » www.theguardian.com