Lie detectors don't work
Lie detectors are not accurate. They are little better than a coin toss.
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Lie detectors measure a candidate’s heart rate, blood pressure, respiration and skin conductivity. The test administrator will ask a set of control questions and measure the candidate’s physiological responses. Then they will ask the key questions of the test and determine to what extent the candidate’s physiological responses change. There are several flaws with this method of deducing truth from lie. The first is that the notion that our physiological outputs change when we are lying is not supported by science. In fact, lying affects different people in different ways. An honest person, telling the truth may display signs of anxiety. Alternatively, an experienced liar may be so accustomed with being deceptive that they are unphased by lying on a polygraph test. The second flaw is that situational factors affect the test's validity. These situational factors, however, are not always identifiable. They can be things like the suspect’s intelligence level, or their natural anxiety levels.  Finally, polygraph tests do not work because there are well-documents countermeasures that allow a subject to “beat” the test. These include subtle physical movements, altering a subjects belief about the test, and the use of agents that alter physiological patterns. 
Even if it isn’t 100% accurate, the polygraph test can be a valuable tool to determine a suspect’s guilt or innocence when considered alongside other forms of evidence. Courts use several methods to determine an individual’s guilt, including witness testimony, expert testimony and cross-examination. None of these methods are 100% accurate at determining a suspect’s guilt. But when they are all employed together, the jury (hopefully), is able to build a picture of the likelihood of whether or not the individual is guilty or innocent. Nobody is suggesting that the polygraph test should be the basis on which a person’s guilt or innocence is determined. However, when used with other methods, it can play a valuable role in building a picture of guilt or innocence. For this reason, lie detector tests should be admissible in court.
[P1] Lie detectors cannot accurately detect an individual's deceit. [P2] Therefore, they should not be admissible in court.
Rejecting the premises
[Rejecting P2] Other methods that are admissible are not 100% accurate Alongside other evidence, lie detectors can be a valuable tool in determining guilt or innocence.