Mental and physical countermeasures reduce the accuracy of polygraph tests.

Effects of countermeasures on the control-question polygraph test were examined in an experiment with 120 Ss recruited from the general community. Ss were given polygraph tests by an examiner who used field techniques. Twenty Ss were innocent, and of the 100 guilty Ss, 80 were trained in the use of either a physical countermeasure (biting the tongue or pressing the toes to the floor) or a mental countermeasure (counting backward by 7) to be applied while control questions were being presented during their examinations. The mental and physical countermeasures were equally effective: Each enabled approximately 50% of the Ss to defeat the polygraph test. The strongest countermeasure effects were observed in the cardiovascular measures. Moreover, the countermeasures were difficult to detect either instrumentally or through observation.