Lie Detector Test – Reliability and training

The polygraph, sometimes (and arguably erroneously) called a lie detector test, is an instrument designed to enable a polygraph examiner to determine the veracity of a person’s statement regarding a specific issue. It does this by measuring selected physiological responses using medical grade instrumentation. These responses are digitally recorded and subsequently analyzed by a highly trained polygraph examiner. The instrument gathers data from three major systems: cardiovascular, respiratory, and endocrine. It monitors and records heart rate, blood pressure and volume, movement of chest cavity, and sweat gland activity. Some record things like arm and leg movement.

To begin a polygraph examination an examiner will attach four to six sensors on the examinee’s body, thereby connecting him or her to the polygraph instrument. These sensors collect physiological responses controlled by the Autonomic Nervous System (ANS). Rate and depth of respiration are measured by pneumographs wrapped around a subject’s chest. Cardiovascular activity is assessed by a blood pressure cuff. Skin conductivity (called the galvanic skin or electrodermal response) is measured through electrodes attached to a subject’s fingertips. The idea is that when a person being examined fears detection, the fear will produce a measurable physiological reaction if the person answers deceptively. The examiner infers deception when the measured physiological responses to questions about relevant questions are different from responses to control questions. The Federation of American Scientists (FAS) argues that polygraph instruments, therefore, measure fear of detection rather than deception. Such criticisms regarding the reliability of polygraph examinations are widespread and worth examining, as are the complications associated with actually becoming certified.


Lie detector tests have been widely criticized. The American Psychological Association (APA) stated in a 2004 report that “there is no evidence that any pattern of physiological reactions is unique to deception.” For example, an honest person may be nervous when answering truthfully and a dishonest person may be completely calm when answering deceptively. The APA notes that there are few studies that validate the ability of polygraph procedures to detect deception, citing Dr. Saxe and Israeli psychologist Gershon Ben-Shahar who suggest that “it may . . . be impossible to conduct a proper validity study.”

The APA reported that the cumulative research evidence suggests that typical polygraph examinations detect deception better than chance, but with significant error rates, both of misclassifying innocent subjects (false positives) and failing to detect guilty individuals (false negatives).

Research on the processes involved in these typical Control Questions Test (CQT) polygraph examinations suggests that several examiner, examinee, and situational factors influence test validity, not to mention the technique used to score polygraph charts. There is little research, however, on the effects of subjects’ differences in such factors as education, intelligence, or level of autonomic arousal, and these also could play a role.

Moreover, evidence indicates that strategies used to “beat” polygraph examinations, so-called countermeasures, may be effective. Countermeasures include simple physical movements, psychological interventions (e.g., manipulating subjects’ beliefs about the test), and the use of pharmacological agents that alter arousal patterns.


Polygraph evidence was largely inadmissible in U.S. courtrooms until 1993 when the Supreme Court decided Daubert v. Merrell Dow Pharmaceuticals, Inc. This case broadened the test for admissibility of expert evidence. It gave judges the freedom to make decisions about whether to admit the evidence of experts, including polygraph examiners, on a case-by-case basis, depending on its relevance, reliability, and the extent to which it meets scientific standards. Although jurisdictions vary in their use of the Daubert principles, polygraph evidence has been allowed in over 20 states and in 9 of the 12 federal circuits.

Nevertheless, U.S. courts are hesitant to admit polygraph evidence into court, as polygraphs remain “nearly universally frowned upon as courtroom lie detectors.” Both state and federal courts tend to express doubt as whether such evidence is reliable. Despite the assurance of due process in U.S. courts, courts have opined that “due process does not require the admissibility of all evidence which may tend to exonerate the defendant,” including polygraphs. A defendant’s right to present relevant evidence is subject to reasonable restrictions, as explained by the court in In re Jordan, for example. The court further clarified that “a polygraph is not so crucial that its absence precludes a defendant from mounting a defense. A defendant can still testify; he can still present corroborating witnesses; he can still cross-examine hostile witnesses.” Over the years courts have vacillated between allowing and barring polygraph evidence, with the Supreme Court ultimately deeming constitutional a military rule of evidence that bars such evidence.

In 1978, a Massachusetts court, in deciding whether to admit polygraph evidence, referred to an earlier decision in which a majority of the court “recognized that a properly conducted test had potential value as an aid to determining whether an individual is telling the truth.” The court emphasized it was a cautious first step toward the acceptance of polygraph testing, and held that the technique had “advanced to the point where it could prove to be of significant value to the criminal trial process….”

In another case nearly 20 years later, however, an Arizona court quoted a Ninth Circuit holding regarding the inadmissibility of polygraph evidence: “if polygraph evidence is being introduced because it was relevant that a polygraph examination was given regardless of the result, then it may be admissible.” That is, if a defendant wanted to introduce polygraph evidence to demonstrate that he could not have committed a crime, because he was taking a polygraph test at the time of the alleged crime, it would probably be admissible.

Finally, in a 1998 case addressing the constitutionality of a particular military rule making polygraph evidence inadmissible in court-martial proceedings, the Supreme Court held that prohibiting polygraph evidence did “not abridge the right of accused members of the military to present a defense.” Because there was “simply no consensus that polygraph evidence is reliable,” a rule barring polygraph evidence in court is constitutional.


The courtroom is not the only place where polygraphs have been barred. In 1998, the United States Congress passed the Employee Polygraph Protection Act, which in effect banned the use of polygraphy for employment screening in the private sector. The ban does not apply to public service employees. Polygraph testing continues to be used in non-judicial settings, often to screen personnel, but sometimes to try to assess the veracity of suspects and witnesses, and to monitor criminal offenders on probation. Many federal agencies, most large police forces, and the military remain major users of polygraphy as a vetting tool.

One might argue that because the government itself uses polygraph testing, the government must consider it reliable. The Scheffer Court, however, clarified that governmental use of polygraph tests is primarily in the field of personnel screening, and to a lesser extent as a tool in criminal and intelligence investigations, but not as evidence at trials because “the foundation of the United States legal system is that the jury is the lie detector.” The Court further explained that unlike other expert witnesses who testify about factual matters outside the jurors’ knowledge—such as the analysis of fingerprints, ballistics, or DNA found at a crime scene—a polygraph expert can supply the jury only with another opinion, in addition to its own, about whether the witness was telling the truth.


Regardless of any controversy surrounding the accuracy of polygraph exams, becoming a certified examiner entails its own set of potential obstacles that should be considered before such an endeavor is undertaken. First of all, polygraph machine prices range anywhere from about $5,000 to $10,000 USD. Because most are shipped from U.S.-based companies, it is also important to note that the U.S. Department of Commerce requires an export license for polygraph system shipments to most countries, including Ukraine. Training can cost anywhere from $2,000 to $15,00022 per person, and demands around 400 to 500 hours, depending on the program and its specific clinical requirements.23 Though training can be conducted at the discounted price of $2,000 in Ukraine, there is no significant discount on equipment purchase. A polygraph exam, if deemed necessary, can be administered by a professional polygraph examiner for around $200.


Due to the controversial nature of polygraph testing, there have been a few alternatives suggested and, in some cases, implemented. These methods, however, tend to present many of the same difficulties as polygraphs. Furthermore, due to the relatively limited practice of these alternative methods, not enough research has been done to warrant the validity of any of the methods. One method involves using fMRI machines. Similar to polygraph testing, the subject is asked a series of question, but rather than tracking external physical responses, investigators can see which parts of the brain are active when the subject answers the question and thus, theoretically, determine the veracity of the subject’s statements. The problem with this is that, besides being cost-prohibitive, the investigators in this scenario would need to be trained medical specialists in order to make such a determination.

Other alternatives include linguistic analysis and voice stress analysis. Not enough research has been conducted on the former, and what little has been conducted appears to have been conducted on English speech patterns. Regarding the latter, efforts to develop meaningful metrics have so far not been successful. Studying a suspect’s demeanor, part of many polygraph programs, is another alternative method. Problems with this and voice stress analysis, however, are similar to those with polygraph testing in that gauging the lying of control groups and criminal suspects can be difficult due to possible distinct reactions between control groups and people suspected of crime.

Attempts to systematize the skill of determining deception have been “disappointing.” In one experiment studying the ability to detect deception from demeanor, United States Secret Service agents averaged 64 percent correct judgments, just 14 percent better than chance. Research on training effects in deception studies show a moderate improvement, but nothing so high as to make it seem a reliable skill which can be taught.


It should be noted that legal psychology programs typically produce legal psychologists, either with or without an accompanying legal degree. That is to say that, like other fields of psychology, legal psychology requires extensive doctoral work, including original research and a dissertation. Most who graduate as legal psychologists, or who graduate with degrees in psychology and law, go on to work in academia or think tanks and are capable of testifying as experts.

Due to the many intersections between law and psychology, however, having an understanding of psychology as a lawyer can be extremely beneficial to lawyers and their clients. Law schools could potentially better prepare their students by offering certain psychology classes, or by incorporating particular aspects of psychology into legal courses. By focusing on particular psychological topics as they relate to the legal world, these classes could better prepare students for their careers as attorneys. For example, rather than becoming polygraph experts, students could learn how to assess evidence, including polygraph test validity. Other examples of legal psychology intersections include conflict resolution and negotiation; judgment and decision-making capacity; prejudice and stereotyping; criminal responsibility; competency; assessment of evidence, including the reliability of eyewitnesses; hedonics; developmental psychology and educational policy; addiction and drug policy. By incorporating these types of subjects into courses, or courses with these topics into the curriculum, a law school could potentially better prepare its students for the legal world.


The scientific community remains extremely polarized about the reliability of polygraph techniques,28 and U.S. courts fairly skeptical. Considering evidence that suggests a variety of factors can influence test validity, including evidence that test takers can manipulate their responses; not to mention arguments that examiners can only share another opinion, this skepticism is understandable. Thus far there has been nothing to indicate any “consensus [among scientists or courts] that polygraph evidence is reliable.” The controversial nature of polygraph examination does not seem to warrant the costs associated with becoming a certified examiner. Moreover, incorporating relevant psychology topics into the law school curriculum seems that it would be much more beneficial to achieving the goals of a legal education.

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