Why polygraphs work

Today, we know that polygraphs are not the lie detectors that we trusted them to be. If there are no universal signals for deception, then it should come as no surprise that there are no fool-proof methods to determine if or when a person is lying; at least, not without requiring the use of additional tools and techniques to investigate further.

The polygraph lie detector works on the same principles as detecting deception through observing behavioral and physiological signs, and it is vulnerable to the same problems. Before beginning the interview or interrogation , the set-up is fairly simple: wires connected to the polygraph machine are attached to the subject or suspect to measure and record physiological changes.

By asking them to simply confirm or deny specific questions with answers already known to the interviewer, any increases or decreases in the amount or rate of perspiration, respiration, and blood pressure can be identified. However, increases in blood pressure or sweating are not, in and of themselves, signs of deception! The most common technique is to demonstrate to the suspect that the machine will be able to tell which card the suspect picks from a deck.

If he is guilty, it is important to make him afraid of being caught. This leads some interviewers to increase their control over the environment, sometimes opting to use a secretly-marked set of cards, essentially relieving the polygraph from having to perform any real deception detection.

The suspect must believe in the ability of the lie catcher. Signs of fear would be ambiguous unless matters can be arranged so that only the liar, not the truth teller, will be afraid. And yet polygraphs are still routinely used by government agencies and law enforcement. This raises an obvious question: Why are they relying on pseudoscience to screen employees and solve cases?

Various versions of polygraph machines were developed by several different American researchers and police investigators over the first few decades of the 20th century.

It began as a device that detected a person's blood pressure, and was later equipped with the ability to measure galvanic skin response on a person's hand which is a proxy for sweat as well as breathing rate and pulse. The idea was that these physiological responses could reliably indicate whether a person was telling the truth or lying, and weren't within his or her control. From the start, though, there wasn't a strong case for why this might be.

Nevertheless, through the s and '60s, investigators developed the testing procedure that's still most widely used today , called the Control Question Technique. Essentially, the questioner will mix control questions vaguely threatening ones that don't pertain to the case at hand , like "Have you ever stolen from a friend?

The subject will also repeatedly be reminded that the machine can accurately distinguish truth from lies, and that it's essential for them to answer truthfully. The idea is that the control questions will arouse some baseline anxiety in response to being interrogated, because the questions are vague and hard to answer entirely truthfully. If they didn't commit the crime in question, the thinking goes, their anxiety would actually be lower for the relevant questions because they'd know they weren't lying.

But if they did commit the crime, these questions would trigger even greater levels of anxiety. All this would be reflected in their physiological responses. So to figure out if someone is lying, you simply compare their physiological responses to the control questions with responses to the relevant ones. If the former are higher, they're innocent. If the latter are, they're guilty. The more practiced you are at lying, the less anxiety is associated with it.

A polygraph test can sometimes be correct, and sometimes be wrong. A principal consideration in the applied sciences concerns the content of a test: what it does, or can be designed to, test. Concealed information polygraph tests, for example, have limited usefulness as a screening device simply because examiners usually cannot create specific questions. There may be exceptions, as in some focused screening applications, as discussed above. Similarly, the import of the test itself must be considered.

For instance, in the judicial context, the concealed information test format might present less concern than the comparison question format, even if they have comparable accuracy. Like a fingerprint found on the murder weapon, knowledge of the scene and, possibly, the circumstances of the crime, is at least one inferential step away from the conclusion that the subject committed the crime. With this test, such an expert opinion would go directly to the credibility of the examinee and thus his or her culpability for the event in question.

This possibility raises still another concern for courts, the possibility that the expert will invade the province of the fact finder. As a practical matter, however, witnesses, and especially experts, regularly comment on the probable veracity of other witnesses, though almost never directly. The line between saying that a witness cannot be believed and that what the witness has said is not believable, is not a bright line.

Courts, in practice, regularly permit experts to tread on credibility matters, especially psychological experts in such areas as repressed memories, post-traumatic stress disorder, and syndromes ranging from the battered woman syndrome to rape trauma syndrome.

This issue is a policy consideration that must be made on the basis of understanding the science well enough to appreciate the quantity of error, and judgment about the qualitative consequences of errors the above discussion of errors and tradeoffs is thus relevant to considerations likely to face a court operating under the Daubert rule. With most forensic science procedures, the criterion is clear. The value of fingerprinting, handwriting identification, firearms identification, and bite marks is closely associated with their ability to accomplish the task of identification.

This is a relatively straightforward assessment. Polygraph tests, however, have been advocated variously as lie detectors and as aids for interrogation. They might indeed be effective for one or the other, or even both. However, these hypotheses have to be separated for purposes of study.

For purposes of science policy, policy makers should be clear about for which use they are approving—or disapproving—polygraphs. Courts have been decidedly more ambivalent toward polygraphs than the other branches of government. Courts do not need lie detectors, since juries already serve this function, a role that is constitutionally mandated.

Many policy makers, lawyers, and judges have little training in science. Moreover, science is not a significant part of the law school curriculum and is not included on state bar exams. Criminal law classes, for the most part, do not cover forensic science or psychological syndromes, and torts classes do not discuss toxicology or epidemiology in analyzing toxic tort cases or product liability.

Most law schools do not offer, much less require, basic classes on statistics or research methodology. In this respect, the law school curriculum has changed little in a century or more. The general acceptance test of admissibility enunciated in the Frye decision expects little scientific sophistication of lawyers or judges. Courts, and presumably juries as well, have thus evaluated expertise based on consensus.

The problem with this test has come in fields that purport to be rigorous but may not be. For instance, if the question is the validity of bite mark identification analysis, researchers who study the. However, if the courts only consider the expert opinions of forensic odontologists who do bite mark identifications for police laboratories, they are unlikely to get a full view of the value of this kind of evidence. Unfortunately, in many fields of forensic science there are no communities of scientists conducting basic research and the only people who are asked as expert witnesses are interested practitioners with little proficiency in scientific methods.

Good forensic science can have salutary results and, in some cases, profound consequences. DNA profiling is a particularly salient example of how good science can be used for both good law enforcement and in the interests of the falsely accused. Lawyers, under the influence of Daubert , are beginning to open their eyes and ears to empirical criticisms of fields long thought settled. In the area of lie detection, good forensic research could directly contribute to national security. In lie detection, for instance, policy makers have not demanded better work, and few scientists have been interested in pursuing the subject.

This powerful combination of ignorance and apathy has, in general, deprived policy makers of good scientific data. More particularly, it has led to convictions of the innocent see Scheck, Neufeld, and Dwyer, , acquittals for the guilty, and numerous costs to individuals, ranging from job loss to social ostracism. Another institutional reality bears mentioning. The law very often asks empirical questions to which there are no scientific answers.

Moreover, while science can take any amount of time to pursue a question and develop an answer, the law has to render a decision in a short time frame. A particularly good example of this is clinical prediction of violence. A large number of legal contexts call for predictions of future violence. These include capital sentencing, parole and pardon hearings, ordinary civil commitment, sexual predator commitments, and community notification laws.

Courts and legislatures have been undeterred by the fact that psychologists and psychiatrists readily admit that science cannot provide such predictions—though the state of the art is improving. For policy makers, the inability to accomplish some task scientifically does not always mean that it cannot be done legally.

In Schall v. The tradeoffs of false positives and false negatives are strikingly different in event-specific and screening applications, primarily because of the great difference in the base rate of guilt in the two settings. Given the very low base rates of major security violations, such as espionage, that almost certainly exist in settings such as the national weapons laboratories, as well as the scientifically plausible accuracy level of polygraph testing, polygraph screening is likely to identify at least hundreds of innocent employees as guilty for each spy or other major security threat correctly identified.

The innocent will be indistinguishable from the guilty by polygraph alone. Consequently, policy makers face this choice: either the decision threshold must be set at such a level that there will be a low probability of catching a spy thereby reducing the number of innocent examinees falsely identified , or investigative resources will have to be expended to investigate hundreds of cases in order to find whether there is indeed one guilty individual or more in a pool of many individuals who have positive polygraph results.

In our judgment, the accuracy of polygraph testing for distinguishing actual or potential security violators from innocent test takers is insufficient to justify reliance on its use in employee security screening in federal agencies.

Although formal benefit-cost analysis might in principle be used to help decision makers evaluate the difficult tradeoffs posed by the use of the polygraph for security screening, in actuality the scientific basis for. Moreover, no scientific basis exists for comparing on a single numerical scale many of the qualitatively different kinds of costs and of benefits that must be considered. The tradeoffs presented by polygraph testing vary with the application. For example, some focused screening applications may present more favorable tradeoffs for polygraph use than those involved in employee security screening in the DOE laboratories.

The quality control program organized by DoDPI and implemented by DOE in its screening activities is impressive in its rigor and the extent to which it has removed various sources of examiner and other variability. Highly reliable polygraph scoring and interpretation, such as these programs aim to provide, are essential if polygraph screening is to have scientific standing.

Reliability, however, is insufficient to establish the validity of the polygraph for screening purposes. The effects of DoDPI efforts to increase reliability on the validity of polygraph screening are untested and unknown.

The primary advances in polygraph technology since the Office of Technology Assessment report have come in the computerization of physiological responses and their display. Computerized polygraph scoring procedures have the potential in theory to increase the accuracy of polygraph testing because they improve the ability to extract and appropriately combine information from features of psychophysiological responses, both obvious and subtle, that may have differing diagnostic values.

However, existing computerized polygraph scoring methods have a purely empirical base and are not backed by validated theory that would justify use of particular measures or features of the polygraph data. Such theory simply does not yet exist. Moreover, existing computerized polygraph scoring methods have not been tested on a sufficient number and variety of examinees after development to generate confidence that their validity is any greater than that of traditional scoring methods.

Although in theory, combining the results of polygraph tests with information from other sources is possible—for example, in serial screening protocols—such approaches have not been seriously investigated. Similarily, evidence on the incremental validity of the polygraph, that is, its ability to add predictive value to what can be achieved by other methods, has not been gathered.

Moreover, the difficulties that exist with computerized scoring of polygraph tests also exist, and may be multi-. Courts following the Daubert rule on admissibility of scientific evidence are likely to look increasingly to scientific validation studies in judging the uses of polygraph data in court.

The existing validation studies have serious limitations. Laboratory test findings on polygraph validity are not a good guide to accuracy in field settings. They are likely to overestimate accuracy in field practice, but by an unknown amount.

The available field studies are also likely to overestimate the accuracy achieved in actual practice. This is the model we used to extrapolate A from reports that provided single sensitivity-specificity combinations see Appendix H. Thus, if there are 10 serious security violators among 10, employees who are polygraphed and the criterion is set to correctly identify 8 of the 10, the test could be expected to erroneously classify as deceptive at least 1,, 3,, 5,, or 5, of the 9, nonviolators, depending on which of the accuracy indexes applied to the test.

Other assumptions about the accuracy and sensitivity of polygraph testing procedures yield similarly dramatic differences between the predictive values of positive test results in screening versus event-specific investigation contexts.

Polygraph testing of suspected Al Qaeda members is different from security screening of federal employees in other ways that should be recognized explicitly.

Problems of language translation and of possible cultural differences in the meanings of deception and truthfulness are likely to create uncertainty in the meaning of polygraph charts and raise questions about whether these tests can be as accurate as similar tests conducted on English-speaking Americans. We note that this criterion was rarely met in the simulation studies that have been used to assess polygraph validity for screening to date.

See United States v. Plaza, F. March 13, ] vacating United States v. Pa Jan. The implications of Daubert for polygraph evidence are not straightforward. Some courts have interpreted Daubert to undermine the per se rule excluding polygraph evidence e.

Posado, 57 F. It is reasonable to expect continued argument in the courts over whether or not the scientific evidence on polygraph testing justifies the use of test results as evidence. The polygraph, often portrayed as a magic mind-reading machine, is still controversial among experts, who continue heated debates about its validity as a lie-detecting device. As the nation takes a fresh look at ways to enhance its security, can the polygraph be considered a useful tool?

The Polygraph and Lie Detection puts the polygraph itself to the test, reviewing and analyzing data about its use in criminal investigation, employment screening, and counter-intelligence. The book addresses the difficulties of measuring polygraph accuracy, the usefulness of the technique for aiding interrogation and for deterrence, and includes potential alternatives—such as voice-stress analysis and brain measurement techniques. Based on feedback from you, our users, we've made some improvements that make it easier than ever to read thousands of publications on our website.

Jump up to the previous page or down to the next one. Also, you can type in a page number and press Enter to go directly to that page in the book. Switch between the Original Pages , where you can read the report as it appeared in print, and Text Pages for the web version, where you can highlight and search the text. To search the entire text of this book, type in your search term here and press Enter.

Ready to take your reading offline? Click here to buy this book in print or download it as a free PDF, if available. Do you enjoy reading reports from the Academies online for free? Sign up for email notifications and we'll let you know about new publications in your areas of interest when they're released. Get This Book. Visit NAP. Looking for other ways to read this? No thanks. Suggested Citation: "7 Uses of Polygraph Tests.

The Polygraph and Lie Detection. Page Share Cite. Quantifying Tradeoffs. The false positive index is FIGURE Comparison of the false positive index and base rate for three sensitivity values of a polygraph test protocol with an accuracy index A of 0.

Recent Policy Recommendations on Polygraph Screening. Making Tradeoffs. Other Potential Uses of Polygraph Tests. Improving Scoring and Interpretation.

Computerized Scoring. The polygraph clearly measures something, because it does all sorts of cool things when you ask people questions. The more practiced you are at lying, the less anxiety is associated with it. Some controlled studie s on polygraph machines have shown that polygraphs are able to distinguish a liar from a truther in some cases, but these tests also misidentified a shocking number of people telling the truth and labeled them as liars.

The National Research Council was quick to point out that these tests are flawed because they are mock crimes, innocent people being accused of real crimes would probably feel much more anxiety than those in a controlled lab experiment.

The list of cons for the use of this test go on , and on , and on. They are a deterrent and in some cases, somebody who believes you can tell when they are lying are more liable to tell the truth.