Updated: Nov 13
A polygraph examination is an analysis of a person’s biological reactions in response to questions. A polygraph machine is used to detect and measure these biological reactions. The data produced as a result of the polygraph examination is analyzed by a polygrapher. Polygraph examination are commonly referred to as lie detector test.
Polygraph Uses in Criminal Law
A polygraph examination is sometimes administered to criminal suspects, criminal defendants, criminal probationers, parolees, or even witnesses in criminal cases. Additionally, polygraph examination are sometimes administered to prospective employees as a condition of employment. This article deals with polygraph examinations as they relate to misdemeanor and felony criminal cases in California.
The Purpose of a Lie Detector Test
The purpose of a polygraph examination is to ascertain whether or not the examined person’s answers in response to questions are consistent with a person who is answering those questions truthfully.
Remember: A polygraph examination does not measure truthfulness; it measures the biological responses that are purportedly related to a person who is truthful or not truthful.
How a Polygraph Machine Works
A polygraph machine works by detecting and measuring a person’s biological response to questions posed to the person examined. The biological measurements include measurements of perspiration, breathing, heart rate (pulse), blood pressure, and more. Some of the more sophisticated polygraph machines purport to measure eye movement and voice fluctuations in response to questions. The changes, if any, in a person’s biology in response to a polygrapher’s questions is supposed to indicate consistency or non-consistency with authenticity, or truthfulness.
Note: Sometimes the polygrapher and the questioner are the same person, but this is not required.
Polygraph Results are Usually Inadmissible Evidence
Per California law, the results of a polygraph examination are not admissible as evidence in criminal court unless the defendant and the district attorney stipulate (agree) that the results are admissible (PC 351). This rule is absolute and makes not exception other than by agreement between the defendant and the district attorney. In fact, clear evidence that proves a defendant submitted to a polygraph examination may not be used as impeachment against the defendant even if the defendant clearly lied about taking the test. In addition, without an agreement between the defendant and the district attorney, none of the following evidence is admissible in criminal court:
Evidence that the defendant offered to take a polygraph examination
Evidence that the defendant refused of failed to take a polygraph examination
Evidence that the defendant submitted to a polygraph examination
The results of a polygraph examination, and
The opinion of the polygrapher
Note: A criminal defendant cannot be forced by anyone to take a polygraph examination. This does not mean that failure to take a polygraph examination is a good option, especially for probationer or parolees who have agreed to submit to polygraph examinations as part of their respective probation or parole conditions. It is important to understand that polygraph testing may be a valid condition of probation, but the court should impose restrictions on the questions (Btoan v. superior Court (2002) 100 CA4th 313). Also, person who is obligated to take a polygraph examination as a condition of probation or parole is probably protected from having the results of that polygraph used as evidence against him or her (See Miranda & Compelled Statements below).
Should a Person Take a Lie Detector Test
Generally speaking, most criminal defense lawyers will advise criminal suspects and defendants to refrain from taking a lie detector test (polygraph test). Every criminal case is different; therefore, whether or not a criminal suspect or defendant should take a lie detector test in his or her criminal case should be decided by the defendant only after consulting with a criminal defense lawyer. With that said, consider the following common circumstances:
Polygraph examination results are not allowed as evidence in criminal court; however, general observation and/or statements made by the defendant before, during, or after the polygraph examination are usually considered admissible evidence against the suspect or criminal defendant, so long as the criminal suspect or defendant has been properly advised of her rights against self-incrimination (Miranda Rights). There are important exceptions to this rule and criminal suspects or defendants should speak to a criminal defense lawyer before agreeing to, or subjecting himself or herself, to a polygraph examination. In addition, the criminal suspect or defendant’s criminal defense attorney should be present before, during, and after the examination to ensure the defendant remains silent when appropriate.
Note: As stated, all statements and observations surrounding the polygraph exam are usually admissible in court even if the results of the polygraph exam itself are not admissible. Criminal defense attorneys should be present during all questioning along with a third person connected to the defense who could testify on behalf of the defendant (usually a defense investigator). This is because attorneys must ensure that the defendant’s rights to remain silent are protected during a polygraph examination, but at the same time, an attorney should not make himself a witness in a case where he represents a party (defendant).
Sometimes, a polygraph examination is administered as part of a condition of probation or parole. Under these circumstances, the probationer or parolee could be in violation of her probation or parole if she does not take the polygraph examination; however, the probationer or parolee should also incorporate the services of a criminal defense attorney to ensure the polygraph examination is administered fairly and within the scope of the probationer or parolee’s probation or parole requirements. Polygraph examination requirements found in probation or parole terms are common where the underlying criminal conviction is related to sex crimes against children, such as lewd acts on a minor, harmful matter sent to a child, possession of child pornography, etc. Keep in mind that a polygraph examination is usually considered compelled when the examination is required by the terms of probation or parole as part of a sex offender management program; therefore, any statements made in connection with the polygraph examination under this scenario should not be used as evidence against the defendant in court.
If a criminal suspect or defendant agrees to take a polygraph test after she speaks to a criminal defense attorney about the issue, then she should also consider whether or not to agree to have the results of that examination used as evidence in criminal court. As stated, if the defendant does not agree to have the results of a polygraph examination used as evidence in criminal court, then those polygraph examination results may not be used; however, if the defendant “passes” the polygraph under this scenario, that great evidence will never be seen by the trier of fact (jury or judge). On the other hand, if the defendant “fails” the polygraph examination under this scenario, law enforcement authorities are likely to redouble their focus, efforts, and resolve to aggressively prosecute the defendant, despite the fact that the results of the polygraph examination may not be used in criminal court. Alternatively, if the criminal suspect or defendant agrees in advance of the polygraph examination to have the results of that examination used as evidence in criminal court then there is usually one of two results: 1) A failed polygraph examination will result in mush easier prosecution against the defendant; 2) a passed polygraph examination will result in either dismissal of the criminal charges, or more difficult prosecution against the defendant.
A criminal suspect or defendant should take a private polygraph examination before she agrees to take a polygraph examination for law enforcement. A private polygraph examination is one administered without the knowledge of law enforcement authorities so that the criminal suspect or defendant can ascertain, in advance, whether or not she should subsequently submit to a polygraph examination that is administered by law enforcement. In fact, a criminal defense attorney who encourages a criminal suspect or defendant to take a polygraph examination without first having that criminal suspect or defendant take a private polygraph examination might be deemed ineffective as an attorney. The fact that a person took a private polygraph examination in advance of a law enforcement administered examination is not likely to remain a secret if she subsequently takes a law enforcement administered examination. This is because the law enforcement polygrapher is likely to ask the criminal suspect or defendant whether or not she has taken a prior polygraph. Despite this, the criminal suspect or defendant should seek the assistance of a criminal defense attorney before she agrees to take a polygraph examination.
Miranda Warning apply to questions posed by law enforcement as part of a polygraph examination (For further detail on this issue, see Miranda Rights).
Unreliability of Polygraph Examinations
Polygraph examination results are not admissible in criminal court absent agreement by the criminal suspect or defendant because polygraph machines are highly unreliable. The error rate for these “lie detectors” is reported to range from ten to thirty percent (10-30%). This means that even the most accurate polygraph machines report inaccurate findings in at least ten percent (10%) of administered tests. Some of the reasons these polygraph machines are unreliable in detecting “truth” include: polygraph machines themselves are subject to human error in administration, the results of the test subject to bias interpretation, there is often ambiguity in the questions posed by the polygrapher, and more.
In addition, the biological responses to questions posed under a polygraph examination could be the result of issues not related to the questioning. For example, nervousness or stress of the person questioned can cause some of the same biological changes that are reported in a polygraph examination. Also, self-inflicted pain during the calibration questioning stage of the polygraph test can influence the results of the actual test question. For example, a criminal suspect who bites her tongue during the polygraph calibration stage (initial innocuous questions used to adjust the sensitivity of the polygraph machine as it relates to the person question) could produce biological responses that affect proper calibration of the polygraph machine.
In fact, evidence suggests that the accuracy of a polygraph examination can be intentionally affected by a person who tries to “trick” or “fool” the polygraph. These tricks include: hypnosis (self-guided physiological response training similar to Lamaze or a “runner’s second wind” [physiological release of pain-releasing endorphins]), training physiological responses to certain questions not related to hypnosis (projecting memories in response to certain questions [similar to method acting]), injury prior to the test (self-inflicted or otherwise), gaining atmosphere familiarity to test or surroundings, settling or changing the level of cognitive dissonance that affects the examined person’s physiology in response to certain questions (training to believe in the justification of an untruthful statement), and more.
Note: A polygraph test is designed to detect "truthfulness" of the tested person. But what a person truly believes is not necessarily the truth. After all, sometimes people believe things to be true that are not in fact true. For example, if a victim truly believes that he was molested by his aunt, but the victim is wrong about that fact due to a false memory, then the victim would likely pass a lie detector test when he states that his aunt molested him. Also, if a witness truly believes that she saw the defendant on a particular day in question, but that witness is factually wrong about what she believe, then the witness is more likely to pass a polygraph examination question on that issue despite the fact that she is not telling the truth.
Finally, a person can be telling the truth and not be truthful at the same time. For example, if the defendant is asked where she was on a particular night in question, the defendant could answer truthfully by responding that she was at her house; however, the defendant could have also been other places other than her house on the night in question.
To learn more about polygraph examinations used in criminal court, or issues related to PC 351, contact our criminal defense lawyers without delay. Our team of experienced criminal defense lawyers have successfully handled hundreds of felony and misdemeanor criminal cases in San Bernardino, Riverside, and Los Angeles County. We have experience with cases involving polygraph examinations and we offer free in-office first-time consultations for criminal suspects and defendant every day of the week. Call today!