The short answer for the questionable polygraph/lie detector test is that it is not admissible as evidence in all cases. It is used as a tool to entice the alleged defendant to 'come clean' and tell the truth. The polygraph simply indicates the rise and fall of specific body functions. Many individuals aren't aware that you cannot be forced to take the test.
Many factors are involved during a polygraph test that can make the test read as if you are telling a lie. You might be nervous about the procedure which results in higher heart rates, respiration, perspiration, and increased blood pressure. This could read a false positive rating that many individuals or officials such as law enforcement are working toward achieving.
The process begins with an interview to discover specific questions as baseline information. Tubes are attached to the person's abdomen and chest complete with a blood pressure cuff and metal plates on your fingertips. At some point, the individual is asked to lie to make a discovery of what happens if the person does lie and the results of the two are compared.
In some cases, if both parties are agreeable, it can be allowed as evidence in states including Arizona, California, Florida, Georgia, and Nevada. However, you must check the restrictions in your area to verify whether it is usable in your location. It is also deemed at the judge's discretion. Federal court systems have a specific set of guidelines used for evidence obtained from the polygraph.
Some jobs use a polygraph test as part of the screening process for high-security positions. If it is used, the person should be coached by an attorney to fully understand the process. The test has been 'banned' in military courts. It is up to the judge in charge of the case to make the decision concerning whether it is allowable by the information provided.
Even though you can refuse to take the test, the police can pressure any suspect into taking the test until other evidence can be located on the crime. It is unfortunate if you pass the test, the authorities may still be convinced you are lying, or you can just 'beat the test.'
It is advised you hire an attorney right away if this should be a factor in your case. You must have legal representation for a defense to defend your legal rights. It is essential you understand how the test works.