The device dubbed the “lie detector” monitors a subject’s four key indicators in an effort to identify deception. The machine’s principle is based upon the supposition that pulse, breathing, perspiration, and electrical conductivity are altered by attempts to deceive, and that a skilled examiner can identify where the relevant graphs reveal untruthful answers to questions posed following a series of standard-setting questions that act as a benchmark. A popular instrument used to screen intelligence personnel in the United States and to test the integrity of sources, it has been rejected by the British intelligence community.
Although the polygraph’s efficacy remains a controversial issue, it is, on the evidence of convicted traitors, a powerful deterrent, and there is some evidence to suggest that vulnerable individuals opt to forgo their security clearances rather than risk failing a test and perhaps thereby prompting an investigation.