A sociopath (read “Obama”) can lie convincingly. No machine can tell the difference.
This is exactly what I thought. A "lie detector" only tells you whether someone is stressed. It doesn't tell you why they are stressed.
Scrupulously honest people have the most difficulty with "lie detectors", because they are stressed about simply being asked if they are being honest.
Sociopaths have no problems with "lie detectors", because they are so practiced at lying, it doesn't stress them.