Because the outcomes of a polygraph test can suggest many matters and are so unreliable in detecting true lies, they do no longer rise to the degree of reliability required for scientific proof in a courtroom and polygraph test consequences are generally inadmissible as evidence.
Polygraphs go by your heart rate, so if the person in question is nervous, it will cause it to spike, saying they are lying, when they could in fact be innocent and didn't commit the crime.
