why dont the police use lie detectors to see if criminals are telling the truth??

Answer 1

Besides the fact that you can beat a lie detector, the constitution says that a person cannot be forced to incriminate themselves. So they have to volunteer to take the test.

Answer 2

Lie detectors are a gamble because they are not always accurate. Many variables can make a lie detector give a false reading.

I would say that it is also to protect the accused. If I were a lawyer, I would not want my client to be subjected to a lie detector test (except for certain circumstances) because it could be used in court against my client. The same applies to the prosecution and the police, it is a gamble to administer the lie detector test if they don't have a strong case because it could help the defendant in the case.

Answer 3

Police do use polygraphs to see if criminals are telling the truth, they use them to determine if they are being deceptive when making certain statements.
There are drawback to using this technique for large numbers of people.
Polygraph tests take an average of 3 hours to complete for each one. The results only show that the person may or not be being deceptive. There is no flashing red light for "lie" and a green one for "truth". The results cannot be used in court (here in Canada) and if the only evidence you have is a polygraph test that says the person is being deceptive and the person is denying guilt, then you won't get a conviction.
Polygraph techs. must take a long and expensive training course and many smaller police services do not have the resource to dedicate a police officer full time for this job. We only have one polygraph technician who is also a detective. He is responsible for his own investigations and tries to squeeze in polygraph request whenever possible.
Polygraphs are generally used as a last resort technique by many officers, especially if you get a situation where two people may be responsible for a crime and you don't have enough evidence to pin point one over the other. Polygraph tests usually include some kind of interview/interrogation after-wards and sometimes people will admit their guilt when confronted with the polygraph results.

Answer 4

they often use a lie detector before/ during a court case. they cant use them under all circumstances because the person reading the lie detector needs to be trained. also, the results are often not accurate. if a person is nervous they are more likely to apparently be lying. any question which aggravates some one will make them sweat more and make it seem like theyre lying. thats why the person using the lie detector generally takes the criminals body language and tone of voice into account while asking questions. also some people can train themselves to lie and for it not to come up on the lie detector. its not that reliable to be honest

Answer 5

The problem with polygraphs is that they don't truly tell you if someone is lying or not. They tell you whether or not the person is displaying the "symptoms" of someone who is lying. Most people, though they do not notice, have physical cahnges in the body functions when they lie. Increased heart rate, increased perspiration, breathing patterns change. It happens on a ver smal scale, but they are detectable. That is what the polygraph monitors. It doesn't know if what they are saying is is a lie or not, it lets the person administering it know they displayed the physcial characteristics of someone telling a lie.

The problem with most criminals, is that their conscience(sp.) doesn't work as normal people's do. They may be able to lie better than the average person. A habitual liar will be able to beat a polygraph, because their body does not react in the same way a normal person's does when they lie.

Also, if you do not know you are lying, then you will not display the signs of a liar. Say your last name is Jones, but I am told it is Smith. I am polygraphed, and they ask me if your last name is Smith. I would ay yes. Though I just lied, I don't know that, and would not show the signs of someone who is lying.

Because of these factors, polygraphs are not admissable in court. They are often used to clear someone's name, but only in certain circumstances.

Answer 6

Because lie dectectors do not have sound evidence for efficacy!! Despite what Trisha Goddard says lie detectors are not 99.9% accurate. Many factors can affect the outcome of a lie detector, and people can beat the lie dectector by using strategies to either conceal their emotions or to disrupt the whole procedure, making their results unreliable. Lie detectors do not tell whether someone is lying or not - they merely report physiological arousal in response to questions asked.

Answer 7

I find it fascinating that in most of these answers comments are made about the "lie detector" not being 100% accurate. Interesting that polygraphs (the actual name for the test) have been found to be at least as accurate, if not moreso, than eyewitness testimony which we routinely rely upon in a court of law with very little question. I would much prefer to rely on a well-administered polygraph examination by a qualified examiner than eyewitness recall any day.

Answer 8

As a former FBI field agent now free lance investigator / lie detector administrator told me, he could devise a set of questions for any circumstance that would "prove" a person either innocent or guilty, whichever he wanted.

The validity of a le detector test is only as good as the person giving the test, wording the questions & analyzing the results. Hence not reliable.

Answer 9

Because the results of the lie detector cannot be used as evidence in court as lie detectors are unreliable.

Answer 10

Because they're not reliable, contrary to what you see on TV.

Lie detectors don't detect lies, they detect things about which a person has concerns.

So, for example, a parent let's their kid out to play and the kid is killed. The parent will feel like they killed the child, and if put on a lie detector and asked, it will read positive. Because they feel responsible.

But lie detectors don't work on serial killers because they are without any remorse or responsibility, they truly don't care. If you ask them "did they kill..." it would not indicate.