Polygraph Testing

March 1, 2012

I received an epistle from the South African Labour Guide yesterday. It was on the subject of polygraph testing and began:

Employers are often faced with dishonest or criminal activities such as misappropriation of property or theft in its work environment, without knowing exactly where, how and by whom it was committed. It is crippling the business and the employer is at its wits-end to find the culprits or wants to know: “Can I send all my employees for a polygraph test and if so, what can I do if they fail?”

It then went on, in answering this question, to miss the point entirely: it didn’t even touch on the scientific validity of polygraph testing. To be fair, the Labour Guide is a guide to labour law, and does not use objective truth as its yardstick; but I would still expect it at least to touch upon one of the best defences against polygraph evidence–that it is pseudoscientific nonsense that has been thoroughly discredited, and whose use should be illegal. It might be instructive to look at how it is supposed to work.

The victim is attached to various wires which connect to a machine that measures heart rate, blood pressure, respiration and skin conductivity. The interrogator will then follow one of two techniques: the control question test (CQT) or the relevant/irrelevant test (RIT). In the IRT the inquisitor will ask irrelevant questions (is your name Fred?), interspersed with relevant questions about which the test is really being conducted (did you plant the bomb?). If the lies (or truth) told in response to the irrelevant show the same physiological responses as those for the relevant questions, then the subject passes the test. The CQT is similar, but additional irrelevant questions are posed that will most probably result in a lie (have you ever lied to avoid trouble?). The assumption is that everyone has lied at some stage in the past, and that if the victim replies to this question in the negative he is lying, which will allow the operator to compare the physiological response to this ‘known lie’ with answers to the relevant questions.
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