lie to me

so is it just me, or is there a real problem with the basic idea of the lie detector, now entering the nightmare phase. ok. so they say this sucker can identify lies with 90% accuracy. however, in order to know that that's the case, you have to know antecedently whether or not the person is lying. for one thing, saying something is false because a "scientist" told you to say something false is not the same situation as one in which you're saying something false in order to deceive someone. but the latter is gonna be a bitch to study, dudes. what you don't want to do is start saying "90%  accurate" on the basis of some horribly problematic methodology, and then subjecting people to this thing. because obviously once deployed it merely confirms itself: you're lying in virtue of the fact that you flunked the lie detector: voila perferct accuracy.