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Thread: How accurate is a polygraph?

  1. #26
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    Quote Originally Posted by Kimster View Post
    If anyone finds a case where someone's poly showed deception and they were found innocent, be sure to post it here.
    Up thread post #13 looks good to me. It's an interesting read.

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  3. #27
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    Quote Originally Posted by HesterMofet View Post
    if polygraphs are so accurate, why aren't they allowed in court? Certainly if they were even as accurate as eyewitness testimony (which is notoriously unreliable), they would be allowed as evidence.

    The website that is mentioned is a pro-polygraph site. I would like to see independent studies on the accuracy of polygraph testing.

    Another issue is that they won't work on true sociopaths/psychopaths who have no guilt over what they have done. If they aren't emotionally heightened or stressed, they won't trip up the polygraph.

    Even if I were clean as a whistle, I would never take a polygraph. Way too many variables and chances for things to go wrong.

    I hate it when I read on here, "He must be guilty! He refused the polygraph!"
    That's just crap.
    IMO it's an emotion indicator, not a lie detector. And as one who works in electronics, I would never risk my freedom on any electronic device.

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  5. #28
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    Quote Originally Posted by Kimster View Post
    "Researchers conducted 12 studies of the validity of field examinations, following 2, 174 field examinations, providing an average accuracy of 98%. Researchers conducted 11 studies involving the reliability of independent analyses of 1,609 sets of charts from field examinations confirmed by independent evidence, providing an average accuracy of 92%. Researchers conducted 41 studies involving the accuracy of 1,787 laboratory simulations of polygraph examinations, producing an average accuracy of 80%."

    http://www.polygraph.org/section/res...idity-research
    I always believe that "surveys" and "research" done by a particular industry about its particular product will result in biased results. If I am a member of the Polygraphy Industry, and I conduct a study, I would never publish reports that showed the inaccuracy of the polygraph. That would be biting the hand that feeds me, that would contribute to a loss of MY income as a polygrapher. I would only publish the results that tended to support that my business is a sound and good one. So, IMO, any research carried about within the industry is biased at best.

    It is interesting to note that this research freely admits that out of each 10 people who are polygraphed, TWO will get results which are NOT ACCURATE. I sure wouldn't want to be one of those two, would you?

    Show me some studies done by independent researchers who do not have a financial interest in how the study turns out, and I might be interested.

    Every polygrapher brings his own subjective bias into the test......if he has a predisposition to believe a subject is guilty, that is what he will see. When two polygraphers can look at the same results and one will believe the subject passed, and the other will believe the subject failed miserably, well something is wrong. When you add to the mix that the polygrapher can lie to the subject, that even further invalidates the test as a scientific one.

    Every Law enforcement officer that I personally know has told me that they would never submit to a polygraph and that they would advice their family members to refuse the test, also. That really says something to me.

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  7. #29
    I agree. This site says "besides, polygraph doesn't detect lies, it detects changes in the physiology of the body, specifically at stimulus (questions)."

    http://www.polygraphplace.com/ubb/No...ML/000219.html

  8. #30
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    I read somewhere that M.I.T. did a study on polygraph examinations and found them to be as accurate as tossing a coin. Newer technology may have increased the accuarcy since that study came out, but it all depends on the expertise of the polygrapher.

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  10. #31
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    Polygraphs have improved over the years and can be of help to investigators to entice a confession. Sometimes, after a person has taken a polygraph and told they failed, they will "give it up."
    The major difficulty, as I see it, with polygraphs, is the way the questions are worded. They are most effective if they are direct, such as "Did you kill (whoever)?" There was a (bad) show on television for a short time that featured polygraphs as the main way to tell if someone was telling the truth or not. The "contestants" could win money and lose their dignity, marriages,... etc. Clearly the questions were designed in the upper levels of the game so any nagging doubt or guilt would show in the polygraph, even though the person was innocent of the transgression. The show inadvertently depicted how people can fail a polygraph.

    But, the end of techniques that deceitful people use is long. All these can throw off a polygraph. Psychopaths can pass them, because they have no conscience, so don't respond with guilt to questions.

    However, now there is a new polygraph, called No Lie MRI. An MRI is taken while the subject is asked very clear cut, yes or no type questions. Questions such as, "Did you kill so-and-so?"; "Did you hire someone to kill...?"; or, "Did you shoot...?". I have more faith in these exams, because they do not rely on physical reactions, but on brain wave patterns. When a person creates a story or lies, a different part of the brain is activated. The MRI detects that. This would also be a huge asset for innocent people. Cases have been dropped when the MRI confirmed what the innocent "suspect" maintained.

  11. #32
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    I'm sure many of you watch these true crime shows. How many times have the cops dismissed a person because they passed a polygraph only to find out years later they were guilty? How many times did they pursue innocent people who failed the test?

    They have discovered that different parts of your brain are active when you are lying vs. when you're recalling information. I can see the polygraph being phased out as they develop machines to scan your brain during questioning. Of course this won't work on a psychopath.

  12. #33
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    How accurate is a polygraph?
    I think that really depends how you're defining accuracy.
    If the question is "how accurate is a polygraph at detecting the physiological signals that it's programmed to detect?" then with modern technology I'd be inclined to believe the "close to 100%" statistic quoted in the OP.

    However, I suspect that the question is really "how accurate is a polygraph at detecting a lie?" and I personally wouldn't put a lot of faith in it.

    As I recall, polygraphs are based on the premise that we're all hardwired to tell the truth and lying prompts a subconscious stress reaction which manifests itself through certain physiological changes such as sweating, changes in heart rate and respiration etc.

    The problem for me is that there are so many variables to take into account. Some people are more predisposed to showing physiological reactions than others (I know a couple of people who visibly blush when under pressure - absolutely no machine needed there to see what's going on) so the degree of variation in vital signs that indicates stress varies from person to person. It's not an exact science where a deviation from the baseline of x is stressed, y is not stressed and z is indeterminate.

    To further muddy the waters, put yourself in the position of an innocent person being wired up to a machine and asked "did you commit this crime?" - I think most of us would show some stress reaction in these circumstances. Ambiguous questions where there's not a straight "yes" or "no" answer can also cause stress while the person makes up his/her mind on what the correct response is.
    So even if a respondent shows a definite stress reaction the interpretation of the results is subjective and dependent on the examiner deciding what's a "normal" stress reaction vs a lie and on their phrasing of the questions.

    If I was falsely accused of something, it's certainly not something that I'd be willing to stake my future on.

    I'll admit to not having read up on it, but in principle I'm not convinced by the MRI method mentioned above either.
    I'll accept that it can identify the area of the brain engaged in a complete fabrication but I have my doubts as to whether it could differentiate between recall of a real event and recall of a script / story that had been memorised in advance. IMO that distinction would very likely also be highly subjective.

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  14. #34
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    The only way a Polygraph Examination could even be remotely helpfu/usefull is 4 other people besides the target take the test at the same time by the same questioner, graded the passed to 5 examiners who pick out the target independently If they can't the test is useless!

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  16. #35
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    I'm probably going to take some heat for this, but here goes anyway. The polygraph is absolutely worthless at determining who is lying and who is telling the truth. It does have some small practicality at narrowing the field, but only insofar as it helps to point in a direction. A polygraph records a persons heart rate, breathing, and what is called the "galvanic skin response" which is sweating. It does these things quite well. What it does not do, and can never do is determine if the person with the elevated heart rate and breathing and sweating has those conditions because they are lying or because they are angry, or nervous. The polygraph operators will assert that they can make that determination, but there is absolutely no science to support that assertion. Every major spying case from the Walkers, and Aldrich Ames, to Robert Hanssen involved the traitors taking multiple polygraphs, and passing with flying colors. In fact, some were concerned about the polygraphs and asked the KGB handlers what to do, and they were told to do nothing, because the polygraph doesn't work. They were told "just don't admit to anything, and continue to assert your innocence." The National Academy of Sciences determined that the majority of polygraph research was "unreliable, unscientific and biased". It concluded that at least 57 of 80 research studies, relied on by the American Polygraph Association as conclusive proof, were significantly flawed. If you want to know more read the book, "Tremor in the Blood" written by Dr. David T. Lykken, a polygrapher himself. In short there is no credible empirical evidence for the validity of polygraphs. Then why does the federal government and the police rely so heavily on them..? That question was posed to a former Director of the FBI, and his answer was that it's better than nothing. The way it works is that in the middle of the polygraph examination, the polygraph operator will stop the exam, and say that he's reading signs of deception in your answers. "Is there anything you'd like to explain before we go any further..?" Then whatever the examinee is coerced into admitting is written up as deception having been discovered by the operator. It is absolute pseudo-science.

  17. #36
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    I don't know how they could ever know for sure how accurate the polygraph is. You can ask test questions and tell the subject to lie and tell the truth and note the differences but in none of those test questions is the thought going to rush through your mind - Gee if I blow this answer, I'm going to the electric chair! Whether you're telling the truth or not, I don't see how that doesn't send the pens bouncing rail to rail.
    This is my opinion and to the best of my knowledge, that is, if I'm not joking.

    Stan Reid

  18. #37
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    The accuracy of polygraph results ranges from 73% to 89% in deceptive subjects when "inconclusives" in large studies were included in the calculations.

    The accuracy of polygraphs ranges from and 91% to 98% when "inconclusives" in polygraph studies on accuracies were excluded from the calculations.
    http://www.stat.cmu.edu/tr/tr766/tr766.pdf

    Dollins, A.B., D.J. Kraphol, D.W. Dutton. 2000. Computer Algorithm Comparison. Polygraph, 29(3).

  19. #38
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    The polygraph is not a "lie detector"!

    I am the only licensed polygraph expert who has ever told the truth about the polygraph, and the truth is, the polygraph is not a "lie detector". I have been telling the truth about the scam called lie detection for almost forty years now in hopes of destroying the dangerous myth of "lie detection". Carl Sagan said, "If it can be destroyed by the truth, it deserves to be destroyed by the truth." I was instrumental in destroying a large part of the polygraph industry by getting most polygraph testing outlawed in the private sector. In 1988, with the passage of the EMPLOYEE POLYGRAPH PROTECTION ACT, administering polygraph tests actually became a federal crime! Even the U.S. Supreme Court refused to admit polygraph results into evidence, and ironically it was the U.S. Justice Department who argued that the polygraph results were not reliable and should not be admitted into evidence! I was a member of the Office of Technology Assessment, (an investigative arm of the U.S. Congress), studying the validity and reliability of the polygraph - our report basically said it was worthless as a "lie detector". I also testified in the U.S. Congress in support of the EPPA. Click here to read a transcript of my testimony: http://babel.hathitrust.org/cgi/pt?i...ew=1up;seq=281 (My testimony begins on pg 275) Here is an interesting piece of historical trivia: When I testified in Congress, I put my manual, HOW TO STING THE POLYGRAPH into the Congressional Record, and the Senators and Representatives distributed more copies of my manual between 1984 and 1988 than anyone has ever distributed - including me! They sent them out by the tens of thousands in response to requests from constituents. But, there were exclusions written into the law that allowed the government - local state and federal - to continue to use the polygraph. They attempt to justify these exclusions on the grounds that the government needs this tool to protect national security and the law enforcement officials need it to protect the integrity of the criminal justice system. I have proved the polygraph is not a "lie detector" - the Congress, the Justice Department, the OTA, and all those with any scientific credibility agree with me - so there is no justification for the government to continue to use it on the pretext that it protects our national security or the integrity of the criminal justice system.

    It is FOOLISH and DANGEROUS to use the polygraph as "lie detector" - the theory of "lie detection" is nothing but junk science. It is based on a faulty scientific premise. The polygraph operators have the audacity to say that there is such a thing as a "reaction indicative of deception", when I can prove that "lying reaction" is simply a nervous reaction commonly referred to as the fight or flight syndrome. In fact, the polygraph is nothing but a psychological billy club that is used to coerce a person into making admissions or confessions. It is FOOLISH and DANGEROUS for government agencies to rely on the polygraph to "test" applicants, or to conduct any type of investigations relating to national security. It is FOOLISH and DANGEROUS for the criminal justice system to rely on an instrument that has been thoroughly discredited to determine whether or not a person is truthful or deceptive, or to use it to guide their investigations in any way - especially when the results cannot even be used as evidence in a court of law! And it is FOOLISH and DANGEROUS for anyone to believe they will pass their polygraph "test" if they just tell the truth! When you factor in all the damage done to people who are falsely branded as liars by these con men and their unconscionable conduct, this fraud of "lie detection" perpetrated by the polygraph industry should be a federal crime! The protection provided to some people by the EPPA should be extended to protect everyone from this insidious Orwellian instrument of torture! Shame on anyone who administers these "tests" - and shame on the government for continuing to allow this state sponsored sadism!

  20. #39
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    Apparently it is very easy to cheat a polygraph test. I was reading my "Criminal Investigation" textbook and it touched up on the polygraph test where in a case, the perpetrator had managed to "pass" the test due to being on tranquilizers. The other suspect in the case, who was deemed the "main suspect" turned out to have failed the polygraph test simply due to being nervous. This mistake was not figured out by detectives until they brought back the perpetrator for further questioning. The perpetrator this time was not on tranquilizers and was arrested after being found out and confessed to the crimes.

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