lie dectors measures reactions. If you know how it works, you can get past it. Lots of natural leagues still have people who cheat. They figure the half lifes of compounds they are illegal, they run it, and they know how to avoid making the detector go nuts. Its not impossible. It also depends on the person doing the lie detector test. I think the competence of the person and his ability to ask and guage in questions also measures the accuracy of the lie detector. Even with the best lie detector examiners im sure that its still like a 95% rate of being accurate. I think an average rate of being correct is fairly low though somewhere in the 60% range when the tester is not that well experienced.