In his column in today’s NYTimes, William Safire mentions that there’s a way, using “a sphincter-muscle trick and a Valium pill” to defeat any polygraph operator. The Valium pill I can understand, but what’s the sphincter trick?
Seems I recall a friend who went through some sort of Lie Detector training in the military discussing this topic. I don’t remember exactly, but it had something to do with tightening the sphincter causing false readings on the test (tight sphincters are not something that I strive to remember).
They were in some sort of embassy guard training and they were being shown how to beat tests for some reason. Hope I’m not giving away national secrets!
If you listen to the people that give the tests, they talk as though it is the most infallible test there is, with no chance of manipulation. I just don’t buy that.
Don’t know about the Valium, thought you were supposed to use amyl nitrate or something 
Since polygraphs bear no more relation to science than the reading of entrails in Ancient Rome, you can tighten or loosen your sphincter with gay abandon, it is still at the whim of the shaman who conducts the test whether to pass or fail you.
National Academy of Science says Polygraph Testing Too Flawed for Security Screening.
I spent a few years as tech support for a company that sold and repaired medical sensors. They are extremely useful for many things. But not detecting lies.
Almost all the tricks you've heard to beat the machine will work. Tighten your sphincter. Stick a tack in your shoe. Alternate between picturing sex with your favorite celebrity, and your body covered by scorpions.
It is also possible to learn to control Galvonic skin response, heart rate, respiration and whatever other readings the machine is taking.
Polygraph tests can not be used as evidence in a court of law. DNA can. One has been proven to be reliable. The other has not.I wonder why they still use it.
Best usage of polygraphs is to intimidate people you suspect are guilty. Give suspect a polygraph, tell him he failed or his answers were “suspicious,” and he’ll get chatty, trying to explain his answers, or sometimes outright confessing.
Polygraphs can actually be extremely useful, if the subject believes it will work. If a person thinks that he’ll be caught if he lies, he might be more inclined to confess. If, on the other hand, the person doesn’t believe in the test, well then, no harm done.
You’ve just got to understand the limits of the test: The results themselves aren’t meaningful, but something meaningful may well come up during questioning.