Deceptive dangers of lie detectors

Deceptive dangers of lie detectors

Britain is likely to mandate from next January the use of the polygraph to monitor sex offenders who are released after serving their sentences.

But its questionable reliability means that polygraph evidence remains inadmissible in British courts, as it also is in Singapore. Here, law enforcement officers have nevertheless resorted to the instrument since 1977 to assess the credibility of suspects and help decide what charges to file, if any.

In some civil cases, people submit reports of polygraph tests they have taken on their own accord, there being a company offering the test here that was set up by a former police officer in 2001.

But the polygraph can't be described as an extremely accurate lie detector. This is because the physiological responses measured don't, in sum, point to a single underlying process of deception.

The instrument measures the examinee's breathing rate, pulse rate, blood pressure and galvanic skin reflex. The last item is based on the fact that moist palms conduct more electricity than dry ones.

The idea is that, on being asked a question to which he plans to lie, the examinee's brain sets off automatic physiological changes that he can't control which the polygraph inexorably captures.

But lying doesn't evoke a unique set of physiological responses. A guilty person's fear of being caught may lead to faster breathing (monitored with belts around the chest and tummy), a quicker pulse and elevated blood pressure (monitored with a blood pressure meter cuff around an arm) and raised skin conductance (monitored at the fingertips).

But a non-guilty examinee's fear of a "false positive" can evoke the same responses too. Nervousness, embarrassment or outrage may cause the same responses as well. Since these alternatives can also explain these measurements in each case apart from lying, what proponents need to establish is a logical explanation or "theory" that explains the link between these physiological responses and guilt (deception).

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