Confirmation bias is a psychological tendency to interpret information in a way that reinforces a preexisting belief. For example, if a suspect believed to be guilty tells the same unchanging story over and over, it must be a rehearsed lie.
Possibly the largest problems with the polygraph, and why it has been inadmissible in most U.S. courts, is the notion that jurors give the test too much weight, particularly in light of its noted inaccuracies.
So why do we lie? Sometimes, it's done to embellish the story about the fish that got away. Sometimes it's intended to shelter someone who would be hurt by the truth (i.e., "Yes, Virginia, there is a Santa Claus.")
If an unimpeachable lie detector expert could testify to his results in a one witness versus another case, determining that the victim was truthful and the accused was lying, or vice versa, he essentially would decide the case instead of the jury.
When Falcon let loose the truth on prime time to Wolf Blitzer, even Barney Fife could have figured it out. By his own admission, that was the first moment the light came on for local Sheriff Jim Alderden.