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About Polygraphs

What is a polygraph test?

A polygraph is a three-part test that measures various physiological processes that can indicate whether a person is telling the truth about an event, experience, or past action.

Can a person beat a polygraph test?

There are ways to mess with the test, but they’re all observable. If a person tries to alter the test, they generally get a warning from the examiner. If the person continues, the test stops and is deceptive by virtue of countermeasures. Countermeasures are methods the investigators use to detect when the person is trying to alter the results of the lie detector.

Trimarco notes, “It’s very, very difficult for a person with no polygraph background to sit down and mislead a former federal examiner or someone who has an education in countermeasures.”

Why would I need/take a lie detector test?

Polygraph testing isn’t just for criminal investigations. There are many situations in which you can ask or be asked to take a polygraph test.

  • Background checks
  • Marital issues
  • Family issues
  • Employee theft
  • Pre-employment testing
  • FBI, CIA, or police application
  • Customs and border protection

How does a polygraph work?

The examination is administered by a trained investigator and can take up to two or three hours, possibly more depending on the scenario. It is comprised of three parts:

Part 1: The Pre-Test

The pre-test is a conversational interview during which the examiner makes a judgment as to whether the person is capable of being polygraphed. The examiner will ask questions involving the person’s amount of sleep, medication use, drug use, and mental psychopathy.

The polygraph pretest interview is intended to orient the examinee to the testing procedures, the purpose of the test, and the investigation target questions. The basic premise of interviewing holds that people will report more useful information when they are prompted to do so by an interested listener who builds rapport through the use of conversation and interview questions. Polygraph pretest interviews are intended to allow truthful examinees to become accustomed to -or habituated to -the cognitive and emotional impact of hearing and responding to test stimulus questions that describe their possible involvement in problematic behaviors, while also sensitizing or increasing the awareness and response potential of deceptive examinees to test questions that describe their past behavior.The polygraph pretest interview is a process, involving several steps (Department of Defense Polygraph Institute, 2002), including: a free-narrative interview, semi-structured interview, or structured interview, a thorough review of the test question stimuli, and a practice or orientation test. The first objective of the pretest interview is to establish a positive identification and introduction, and to clarify the roles of the examiner and examinee. The examiner will also introduce the examinee to the examination room, including the use of audio or video recording devices, and all of the polygraph sensors that will later be attached to the examinee. The next stage of the process consists of making an initial determination of the suitability of the examinee and obtaining informed consent for testing. This is done after a review of the rights of the examinee during testing, including the right to terminate the examination at any time. Ideally, informed consent should also include information about who will receive the information and results from the examination, and where to obtain more information about the strengths and limitations of the polygraph procedure. The examiner will then engage the examinee in a brief discussion about the case background and personal background of the examinee, in order to continue to establish an adequate and suitable testing rapport. The examiner will also provide more information about the psychological and physiological basis for the polygraph test, and will provide answers to any questions the examinee may have regarding the testing procedures.

A practice test or acquaintance test should be conducted as part of standardized field practice. The purpose of this test is to orient the examinee to the testing procedure before commencing the actual examination.


Part 2: The Test

During the test portion, a physiological recorder measures three bodily indicators that reveal when a person lies:

Heart Rate/Blood Pressure

Cardiovascular activity is measured by a blood pressure cuff.


Breathing patterns are captured by pneumographs wrapped around the subject’s chest.

Skin Conductivity/Perspiration

Perspiration is documented via electrodes attached to the fingertips.

While the polygraph measures reactions, the examiner will ask a series of questions. Sometimes, the same question will be asked multiple times in the search for consistent significant reaction or lack thereof. The questions include control questions that set a base for relevant questions, which are directly related to the reasons for taking the polygraph. In theory, a truthful person will react more significantly to the control questions than the relevant questions.

Note: A polygraph measures what a person believes to be the truth, not necessarily what actually happened.


Part 3: The Post-Test

Once the test is complete, examiners measure the reactions and come to one of three possible conclusions:

  1. Telling the truth
  2. Not telling the truth
  3. No opinion

What does a “no opinion” result mean?

No opinion, formerly referred to as inconclusive, means that the investigator could not determine whether or not the person was telling the truth. About 10% of lie detector tests result in no opinion.

How accurate is a polygraph?

The accuracy of polygraph testing is a highly debated subject mostly because human reactions are not standardized. An honest person may act nervous while a deceptive person might maintain calm. There’s also little research that examines whether outside factors, like education or intelligence level, affect the test.

However, in 2002 the National Academy of Sciences conducted a comprehensive review that concluded polygraph testing to be better than chance even if it fell short of complete accuracy. Raymond Nelson, president of the American Polygraph Association, claims the accuracy rating is above 80%.