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Simple English definitions for legal terms
polygraph
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A quick definition of polygraph:
A polygraph, also known as a lie detector test, is a machine that measures a person's physical responses when they answer questions. It checks things like heart rate, blood pressure, and skin conductivity to determine if someone is telling the truth. However, the test is not always accurate and cannot actually detect honesty. It is often used to try and prove if someone committed a crime, but it is not admissible as evidence in court. During the test, the person being tested is hooked up to devices and asked questions, including control questions and relevant questions. The test is based on the idea that an innocent person will be more agitated answering control questions than relevant questions. Another type of test, called the Guilty Knowledge test, asks multiple-choice questions that only someone who committed the crime would know the answer to.
A more thorough explanation:
A polygraph, also known as a lie detector test, is a machine that measures a person's physical responses when they answer questions. The test measures a person's heart rate, blood pressure, respiration, and skin conductivity. The purpose of the test is usually to determine whether or not a person committed a crime.
However, the polygraph test cannot actually test for honesty. Instead, it relies on the polygraph operator's analysis of the tested person's responses, which can be inaccurate. As a result, polygraphs are usually not admissible as evidence in United States courts.
The polygraph test begins by attaching devices such as pneumographs, blood pressure cuffs, and electrodes to a person's body. The person is then told how the test works and asked some sample questions. Next, the tester will use a technique such as the Control Question Test (CQT) or the Guilty Knowledge test (GKT).
Under the CQT, the person answers both control and relevant questions. Control questions are broader and pertain to a person's past, while relevant questions pertain to the crime a person is under suspicion of committing. The underlying assumption is that a person innocent of suspected crime will be more agitated answering the control questions than the relevant questions.
During the GKT, a person answers a multiple-choice test that contains answers that only a person who committed the crime would know. The thought behind this test is that the correct answer out of a lineup would cause a greater reaction from a person who was guilty, whereas an innocent person's response would not differ.
John was suspected of stealing money from his workplace. To prove his innocence, he was asked to take a polygraph test. During the test, John's heart rate and blood pressure increased when he answered the control questions, which were about his past behavior. However, his responses to the relevant questions, which were about the crime he was accused of, did not cause a significant reaction. Based on this, the polygraph operator concluded that John was telling the truth and did not steal the money.