For whatever it's worth (and I usually charge quite a bit for it), take my advice: never, ever, ever, ever take a polygraph test. Besides being junk science, the police don't even use them for the purpose of helping them investigate. They are used for interrogation purposes, i.e., to apply psychological pressure to somebody they
already believe (not necessarily based on any reliable evidence) to be guilty to get them to confess. When you're at a police station and you're asked to take a polygraph, you've already failed it.
This stuff has been studied, although hardly anybody is familiar with the research.
http://www.amazon.com/dp/0674035313/?tag=neogaf0e-20
Every American should read that book.