A polygraph (popularly referred to as a lie detector) measures and records several physiological indices such as blood pressure, pulse, respiration, and skin conductivity while the subject is asked and answers a series of questions. The belief is that deceptive answers will produce physiological responses that can be differentiated from those associated with non-deceptive answers.
According to the American Polygraph Association over 250 studies have been conducted on the accuracy of polygraph testing during the past 25 years. Recent research claims that the accuracy of the new computerized polygraph system is close to 100%.
The Office of Technology Assessment, OTA, the official science advisors to Congress, disagreed with these claims:
The wide variability of results from both prior research reviews and OTA’S own review of individual studies makes it impossible to determine a specific overall quantitative measure of polygraph validity.
These opinions are echoed by the National Academy of Sciences (NAS):
The NAS has extrapolated that if the test were sensitive enough to detect 80% of spies (a level of accuracy which it did not assume), this would hardly be sufficient. Let us take for example a hypothetical polygraph screening of a body of 10,000 employees among which are 10 spies. With an 80% success rate, the polygraph test would show that 8 spies and 1,992 non-spies fail the test. Thus, roughly 99.6 percent of positives (those failing the test) would be false positives.
The accuracy of the polygraph has been contested almost since the introduction of the device. In 2003, the NAS found that the majority of polygraph research was “unreliable, unscientific and biased“, concluding that 57 of the approximately 80 research studies that the American Polygraph Association relies on to come to their conclusions were significantly flawed. These studies did show that specific-incident polygraph testing, in a person untrained in counter-measures, could discern the truth at “a level greater than chance, yet short of perfection”.
The OTA and NAS are joined by The Journal of Forensic Psychology Practice, The Journal of Applied Psychology, The American Psychological Association, and many others. They all agree that polygraphy is poorly structured, hard to standardize, subjective, vulnerable to countermeasures, and heavily biased. In retrospect, this is an obvious conclusion. Red flags should go up whenever your biggest supporter is Maury Povich.
The Basis of Polygraphy
The physiological responses measured by the polygraph are not uniquely related to deception. That is, the responses measured by the polygraph do not all reflect a single underlying process (deception). A variety of psychological and physiological processes, including some that can be consciously controlled, can affect polygraph measures and test results. Moreover, most polygraph testing procedures allow for uncontrolled variation in test administration (e.g., creation of the emotional climate, selecting questions) that can be expected to result in variations in accuracy and that limit the level of accuracy that can be consistently achieved.
Conclusion: Almost a century of research in scientific psychology and physiology provides little basis for the expectation that a polygraph test could have extremely high accuracy.
Theoretical Basis: The theoretical rationale for the polygraph is quite weak, especially in terms of fear, arousal, or other emotional states that are triggered in response to relevant or comparison questions. The idea that certain body states (sweating, heart rate, etc.) reflect only a single psychological process (lying) has not been supported by the evidence.
Conclusion: There is some truth to the idea that certain body states may correlate with deception, but the evidence does not allow for this to assume causation or a high degree of accuracy.
Research Progress: Polygraph research has proceeded in relative isolation from related fields of basic science and has benefited little from conceptual, theoretical, and technological advances in those fields that are relevant to the psychophysiological detection of deception.
Conclusion: Research on the polygraph has not progressed over time in the manner of a typical scientific field. It has not accumulated knowledge or strengthened its scientific underpinnings in any significant manner. This is a key indication for pseudo-scientific modalities. When a field of science does not incorporate new evidence, techniques, and theories–and proceeds without them–the field becomes pseudoscience.
Polygraph Accuracy: Accuracy may be highly variable across situations. The evidence does not allow any precise quantitative estimate of polygraph accuracy or provide confidence that accuracy is stable across personality types, sociodemographic groups, psychological and medical conditions, examiner and examinee expectancies, or ways of administering the test and selecting questions. In particular, the evidence does not provide confidence that polygraph accuracy is robust against potential countermeasures. And there is essentially no evidence on the incremental validity of polygraph testing, that is, its ability to add predictive value to that which can be achieved by other methods.
Conclusion: There is no evidence for sustained accuracy for polygraph results.
Utility of the Polygraph: There is substantial anecdotal evidence that admissions and confessions occur in polygraph examinations, but no direct scientific evidence assessing the utility of the polygraph. Indirect evidence supports the idea that a technique will exhibit utility effects if examinees and the public believe that there is a high likelihood of a deceptive person being detected and that the costs of being judged deceptive are substantial. But any technique about which people hold such beliefs is likely to exhibit utility, whether or not it is valid.
Conclusion: The only scientific evidence that the polygraph test may have some use shows that people must think it will work for it to work. Any utility that we get out of the polygraph is then based on placebo, and scientifically useless.
Effectiveness: All of the physiological indicators measured by the polygraph can be altered by conscious efforts through cognitive or physical means, and there is enough empirical research to justify concern that successful countermeasures may be learnable.
Conclusion: Basic science and polygraph research give reason for concern that polygraph test accuracy may be degraded by countermeasures, particularly when used by major security threats who have a strong incentive and sufficient resources to use them effectively. If these measures are effective, they could seriously undermine any value of polygraph security screening.
Via the National Academies Press:
We have reviewed the scientific evidence on the polygraph with the goal of assessing its validity for security uses, especially those involving the screening of substantial numbers of government employees. Overall, the evidence is scanty and scientifically weak. Our conclusions are necessarily based on the far from satisfactory body of evidence on polygraph accuracy, as well as basic knowledge about the physiological responses the polygraph measures.
Overall, the science does not support the claim that polygraph test are reliably accurate detectors of deception. Because they rely on indirect indicators of deception (heart rate, blood pressure, respiratory rate and sweat, in comparison to normal levels) they may just be picking up on other emotional cues. For example, fear of the polygraph tester himself (let’s say that he says he will kill you if you don’t take the test) will elicit many of the many indirect indicators, and may make it seem to the machine that you are lying. In short, because there is no direct measure of deception (yet), there is not a reliable basis for the method and accuracy of polygraphy.
We may one day be able to directly measure the indicators of deception, possibly with FMRI brain imaging, but so far there are only the peripherals of various emotional responses.
Despite all of this refutation, reports and all, polygraph results are still admissible in court in many developed countries.
It has not accumulated knowledge or strengthened its scientific underpinnings in any significant manner. This is a key indication for pseudo-scientific modalities.
The foundations for polygraphy are flawed, the evidence is lacking, and the science disagrees. Therefore: science cannot yet tell, with any significant accuracy, if you are lying.