Why aren`t the polygraph tests accepted as a proof in a trial ?

Answer 1

In some states, New Mexico, for example, they are permitted. And in others, they may be considered on a case by case basis. Actually, the accuracy rates of the results of a well-performed polygraph by a licensed examiner run conservatively at 65% or so. Interesting that this rate likely exceeds the accuracy rate of eyewitness testimony, yet we routinely allow eyewitness testimony without question! Courts have never required a 100% accuracy rate for evidence to be allowed, not even DNA can boast that rate.

Further, people generally do not know how polygraph exams are conducted. First, they don't ask you outright "Did you murder so-and-so." There are several types of tests that can be done. The Control Question technique, The Relevant-Irrelevant Test, and the Guilty Knowledge Test are a few examples. My favorite is the guilty knowledge test and it is probably the most reliable. It relies on the fact that only the perpetrator or someone involved would know certain things about the crime. Say, for example, a red hat was found at the crime scene and this knowledge was kept out of the press. During the polygraph, the examiner asks if you own a red hat. Well, an innocent person who DOES own a red hat could respond "yes" and not get physiologically aroused (elevation in heart rate, respiration, blood pressure and GSR), while the guilty person might answer "yes" which is the truth, but in so doing get physiologically aroused because he knows this is relevant info to the crime.

Polygraphs measure CHANGES in heart rate, respiration, blood pressure and galvanic skin response (palm sweating) patterns over different types of questions, not just response to questions. The timing of these changes vis-a-vis the question is what is important. Thus, someone who is generally nervous might show even greater increase when asked "guilty" questions. There are techniques to account for nearly all of the phenomena described as "beating the test."

Answer 2

Polygraphs are strictly a tool. It gives the investigator information that reflects deception on the part of the person who was the guest of honor in the chair.
This is why at the start of a polygraph they ask you a series of general questions and toss in some that they know you'll lie to. Then they watch the needles that measure blood pressure, heartbeat and breathing. Ever lie and feel your face flush? Thats your bloodpressure. It shows clearly on the scale as taking a jump on the chart.
However they are unreliable as some young lady indicated before they can be unreliable. If you're questioning a real psycho and the person actually believes in their minds that what they did was right you'll probably get no indication on the chart.
Some people are naturally nervous and being hooked up to a machine and questioned by the cops may send them into the middle of next week. So...I prefer neurolingustics. Abit more accurate and lots more fun to work on someone with. Involuntary reactions with your eyes. Couple that with reading body language and you can question someone, find that one thing they may be lying about and with alittle talent pin them down and move in for the kill.

Answer 3

It seems to be the public's general opinion that the results of polygraph testing is not allowed in court under any circumstances.

The truth is that Polygraph results are admissible in most courts across the country. The Supreme Court has yet to rule on the issue of admissibility so it has been up to individual jurisdictions to allow or disallow them. There are some jurisdictions that have absolute bans on admitting polygraph results, but most allow them.

The simple fact is that both the plaintiff and the defendant have to agree to having the results of the test be admissible, prior to the examination being conducted.

Since the results of the test are going to hurt one party and help the other, the likelihood that both parties will agree to admissibility before knowing how it will affect their case, is minimal. Because of this, results of Polygraph testing are rarely admitted as evidence.

Answer 4

Actually, some places (like Singapore) do accept the polygraph test evidence at trial.

Polygraph tests only say whether the person is being deceptive or not. So if you ask "did you kill him?" and they answer no and the machine shows deception, it only proves they are being deceptive. It does not means that they meant to answer yes.
Do you see the difference? It also will vary according to the skill of the polygraph technician.
Many people will be answering here that you can beat a polygraph test, but I can tell you that you can not control your basic physiological responses to stress, heart-rate, blood pressure. If you are deceptive, it will show, no matter what you do to try to fool the machine.

Answer 5

many of the answers I've read are applicable to some degree.
When they speak of reliability is the polygraph more our less reliable than a witness under oath ?
Actually, the defense may introduce the results of a polygraph in trial and the jury or judge may give it the weight they decide too.
The prosecution can not introduce polygraph results in a criminal trial because the defendant/s are protected from self-incrimination. A defendant does not even have to testify in a criminal case if he or she so chooses.
It is the duty of the prosecution to present the burden of truth, the defense merely has to provide reasonable doubt.

Answer 6

Polygraphs can be manipulated by a number of ways. I've even heard of people sticking pebbles in their shoes to cause a pressure when responding truthfully to a question. Then they relax when they want to not tell the truth. That causes a flip-flop on the graphs, which can make the test inaccurate. A person can control the tests by breathing techniques. It's not very accurate if you know how to work the system.

Answer 7

The results can be manipulated. If you can control your emotions, you can beat a polygraph. Also, if you are a sociopath or psychopath and feel no sense of guilt, the polygraph doesn't work.

Answer 8

Because they are only as good as the person giving the test. If you were innocent but because of the question phrasing you reacted to it some might interpret that as a lie. So you have question phrasing, interpretations and mixed results effecting the accuracy.

Answer 9

They are not reliable. A pathological liar can beat the thing every time, because he believes his lies are the truth, and hardly any physiological reaction will be registered on the machine, whereas a nervous guy who may be telling the truth will be label a liar.

Answer 10

I believe that some factors will result in a false reading.If you are on medications,or if you have a neurological disease it will affect the reading.