Nope, they don’t. All they detect is your level of excitement (pulse, sweating, etc). In fact, many psychologists and scientists are vehemently opposed to the use of polygraphs in employment because they are so poor at detecting lies, and the results stay with the individual for quite some time.

Note: oftentimes, practiced liars have an easier time passing a polygraph than a squeaky clean choir boy. Why? Because compulsive liars and psychopaths are used to lying. The choir boy, on the other hand, would get nervous if you even question his motives.