Asked by: Anonymous
Because they don’t work well enough. Lie detectors have a long history, including pouring molten metal onto suspects’ hands (lying can increase sweating, and sweaty hands burn more) and keeping grains of rice in the mouth (lying can reduce saliva, so dry rice is incriminating). Modern lie detectors, such as the polygraph, include scientific measures of salivation, blood pressure, pulse and muscular activity as well as sweating, measured by the ‘galvanic skin response’ using electrodes on the skin.
There is no doubt that measurable changes happen when people lie, and correlations as high as 90 per cent have been claimed for some methods of detecting these changes, but even this is not good enough to be sure you are convicting the right person. There are also many ways of training yourself to pass lie detector tests by controlling your own physiology. This is why many people think that using a polygraph is as bad as dunking putative witches in a pond.