The polygraph has long captured the public’s imagination. For decades, polygraph tests were used in TV shows and movies to detect lies. What happens behind the scenes? How does a polygraph work and how are its results analyzed?
Lie detector tests are built on the theory that lying produces measurable physiological changes. Polygraphs measure and record changes in blood pressure, pulse, respiration, and skin conductivity while asking multiple questions. The assumption is that the stress of lying will produce different responses than answering truthfully. Polygraph tests follow a structured format. Before the test, the examiner will review the questions in advance and get a sense of the person’s normal physiological responses. During the test itself, the examiner attaches sensors to measure the physiological responses mentioned above. The questions alternate between relevant questions related to the issue under investigation and neutral control questions about other matters. After the test is complete, the examiner analyzes the physiological data to try to detect significant changes in response to relevant vs. neutral questions.
Analyzing the data
Once the polygraph test is complete, the examiner begins carefully analyzing the data recorded. This analysis happens both during the test itself, as the examiner observes the subject’s reactions in real-time, and afterward, as the examiner reviews the detailed measurements. For each question asked, the examiner assigns a score based on the strength of the reaction observed. The scores on relevant questions are then compared to the scores on neutral questions. If there is a substantial difference, it may indicate deception.
Examiners also look closely at the graphical output of the lie detector test locations across USA. It shows the subject’s physiological measures of jumping and falling throughout the test. Examiners look for significant spikes or changes at key moments that may indicate a stress reaction to a particular question. However, polygraph results are not as simple as “lie” or “truth”. The American Polygraph Association advises against using polygraph testing for “yes” or “no” binary decisions. Rather, polygraph tests are most accurate when used along with other evidence to explore probabilities and likelihoods. The physiological data gives the examiner insight into reactions that may warrant further investigation.
Pitfalls and limitations
While polygraphs remain a useful investigative tool, there are pitfalls examiners must be aware of to avoid inaccurate conclusions. One potential issue is the “guilty knowledge” test, in which examiners try to determine if the subject has inside information about a crime. This test sometimes misread an innocent person’s anxiety as deception.
Certain individuals may also be difficult to read accurately on a polygraph. Sociopaths and psychopaths may show muted reactions to lying due to a lack of anxiety or remorse. Other health conditions and medications also alter results. Polygraph examiners need to take the subject’s background into account. Countermeasures are actions taken to try to “beat” the polygraph. Techniques include altering breathing patterns, inflicting pain to alter reactions, or taking medications to decrease anxiety. Examiners have to remain alert to detect any attempts to manipulate results.