Polygraph tests aren’t admissible in court for a reason.
> In the 1998 US Supreme Court case United States v. Scheffer, the majority stated that "There is simply no consensus that polygraph evidence is reliable [...] Unlike other expert witnesses who testify about factual matters outside the jurors' knowledge, such as the analysis of fingerprints, ballistics, or DNA found at a crime scene, a polygraph expert can supply the jury only with another opinion."[25] The Supreme Court summarized their findings by stating that the use of polygraph was "little better than could be obtained by the toss of a coin."[25] In 2005, the 11th Circuit Court of Appeals stated that "polygraphy did not enjoy general acceptance from the scientific community".[26]
Polygraph tests aren’t admissible in court for a reason.
> In the 1998 US Supreme Court case United States v. Scheffer, the majority stated that "There is simply no consensus that polygraph evidence is reliable [...] Unlike other expert witnesses who testify about factual matters outside the jurors' knowledge, such as the analysis of fingerprints, ballistics, or DNA found at a crime scene, a polygraph expert can supply the jury only with another opinion."[25] The Supreme Court summarized their findings by stating that the use of polygraph was "little better than could be obtained by the toss of a coin."[25] In 2005, the 11th Circuit Court of Appeals stated that "polygraphy did not enjoy general acceptance from the scientific community".[26]
https://en.wikipedia.org/wiki/Polygraph#Effectiveness
A polygraph is based on measuring normal emotional response.
A psychopath could probably pass a polygraph without any problem since they are not exactly normal.