Lie detector tests are not 100 percent effective. Different people have different reactions in certain situations, so it's impossible to say with certainty whether someone is telling the truth. Lie detector tests should not be allowed in criminal trials because innocent people could be found guilty, or guilty people could be found innocent.
The lie detector works on the premise that any sort of untruth will trigger a physical response in the test-taker. If a person has been raised to do so, great. However, what if we are dealing with a person whose psychology doesn't react to lies? If a person puts themselves in pain for the duration of the test then the physical baseline for truth will be off. If a person believes that a lie is the truth, they will also misdirect the test-givers. There are too many variables in the test that cannot be accounted for. Can the test be useful? Yes. Is there any way to know? Not really. Lie detector tests should not be used.
I believe that lie detector tests should indeed be outlawed. The highest courts in the United States have determined and upheld the opinion that polygraphs are inaccurate to the point that their results, or even the mentioning of administering a polygraph, are inadmissable as evidence. However, law enforcement still us them to assist in investigations. A method that has been proved to be inaccurate should not be a factor in a criminal investigation as the rights of citizens could be infringed upon due to faulty science.
Lie detector tests should not be outlawed, but their results need finer interpretation by examiners. The tests can be defeated by people who have trained themselves to control their reactions, for example, sociopaths, career criminals and CIA agents. Average people or even petty criminals will not be able to defeat the lie detector test.