Who hasn’t watched a crime movie or drama without seeing a polygraph? The lie detector test and the polygraph machine have become all too familiar in today’s pop culture. They are no longer just used in criminal investigations but also in movies, dramas, and even game shows.
A lie detector test, also known as a polygraph, is a device used to measure, detect, and record physiological indicators such as skin conductivity, pressure, respiration, and pulse while a person is subjected to several questions prepared beforehand. It was invented by a University of California-Berkeley medical student John Augustus Larson and a Berkley police officer in 1921.

The Scientific Basis for Polygraph
According to a research conducted by Raymond Nelson in 2015, there is evidence to prove the validity of polygraph tests in terms of screening and diagnostic contexts. Nelson furthermore asserted that polygraph tests combine physiological responses and response differences depending on the types of stimuli.
A polygraph test is fondly called as a ‘lie detector test.’ However, according to Nelson, the test does not really try to identify lies. More accurately, the results indicate probability. The results of the tests are measurements of uncertainty and are based on the differences between the responses made to the questions asked.

Types of Questions Asked During a Polygraph
There are four types of questions used during polygraph tests. They are control questions, relevant questions, concealed information questions, and irrelevant questions. Control questions and relevant questions are often asked alternately. The relevant questions are those connected to the topic under investigation. While control questions are asked in a more general tone. For example, if the crime was murder, the control question would be, ‘Did you kill the deceased?’ The relevant question would be, ‘Have you ever killed anyone before?’
On the other hand, irrelevant questions are those not pertaining to the investigation. They are often used to put the person at ease and get normative values. Concealed information questions are those asked to illicit detailed responses with regard to the crime committed.
These questions are skillfully crafted in order to illicit answers without the person realizing it. The examiner has to be skillful in carefully picking which type of question to ask to get accurate results.

Types of Tests in a Polygraph
Aside from the various types of questions asked during a polygraph test, there are also several types of tests conducted using the polygraph machine. This article will discuss in-depth the following tests: stimulation test, concealed information test, peak-of-tension test, and comparison question (control question test).
The stimulation test, also known as acquaintance test and stim test, is used as a pretest or as a test conducted between the charts. The persons subjected to the polygraph tests are given several questions about similar items. They will be asked to lie in one of the questions. The purpose of this test is to observe the differences in machine readings with each question. Some examiners use playing cards and ask the examinee to pick a card. They will be asked to deny the number of the card they picked while hooked to the machine.
The second type of test is the concealed information test. Also known as the concealed knowledge test or guilty knowledge test, the examinees are again presented with similar items like in the stimulation test. However, there will be several false details and one true detail with regard to the items given. Only the examiner knows the true answer.

The guilty examines would likely reveal the concealed knowledge by responding more strongly to those items they perceive to be true than other items they know are false. Concealed information tests only apply to certain conditions. There must be a specific activity, incident, or thing as topic of questioning.

A peak-of-tension test resembles the format of a concealed information test. But what makes it different is that the questions are formatted in a specific order. The guilt of the examinee is determined when he demonstrates a certain pattern of behavior when presented a series of options that include the correct answer. His reactions are supposed to intensify as the options near the correct answer and decrease after it has been mentioned. This test is used when the investigator or examiner is still in search of answers. This pattern as indicated in the polygraph machine printouts will reveal the correct answer to the examiners.
Comparison question tests compare the responses of the examinee in the various types of questions to find a correlation, a pattern, or a remarkable indicator of truth or deception.

Significance of Polygraph or Lie Detector Tests

Today, polygraphs are used in police investigations in the United States. In a US Supreme Court decision, the admission of the results of a polygraph test in trial was left to the jurisdiction of the individual courts. Law enforcement agencies, prosecutors, and defense attorneys of a lot of states still use it. New Mexico, in particular, admits polygraph test results in jury trials under certain circumstances.
As of 2013, there were 70,000 individuals who went under the polygraph test as requested by the federal government. The US National Security Agency (NSA) even released a video entitled ‘The Truth about the Polygraph” in 2010. The video explains the whole process of the polygraph test.