They don’t. Unless you count giving a Lebanese Hizballah illegal a security clearance, which might have been due to something other than the polygraph but is ha

They don’t. Unless you count giving a Lebanese Hizballah illegal a security clearance, which might have been due to something other than the polygraph but is hard to reconcile with any background check actually being performed. Polygraphs could work as advertised for 99.99% of people being processed for security clearance and still be useless, because their effectiveness against people who take the job with the intention of leaking, selling, or otherwise inappropriately handling classified information is nil. The US government has spent tons of money on polygraph research and administration over the last century despite knowing from the beginning that it was a fraud, and has never even once prevented an act of espionage with one.