a polygraph only registers your Autonomic Nervous System (ANS) responses. it is prone to what is known as the 'Othello Error'. Essentially Othello error occurs "when the lie catcher fails to consider that a truthful person who is under stress may appear to be lying." (Ekman, 1985
). A polygraph has no ability to account for context. if a question is raised about someone breaching a condition, the offender may remember doing it in the past and the involuntary physiological responses will cause the polygraph to spike, even though the question may only refer to the offenders current condition or situation. it only sees emotional change.
Basically, polygraphs can only register changes in respiration, temperature, perspiration rate, and heart rate / blood pressure - NOT whether a person is being deceitful or not. I would not like my future determined by the results of one of these machines. the machine cannot differentiate between detection apprehension and fear of disbelief. that is to say, if you are worried you might not be believed when giving an answer, the machine can record the same reading as someone who is worried about being caught out. heart rate would increase, temperature will fluctuate and perspiration / respiration levels will likely increase. the polygraph really isn't flawless - not by a long way.
it is common practice for polygraph operators or officials to let people believe that it is a brilliant lie catcher and that it will catch you out, but in truth it really can't. It may aid an investigator in which areas to probe more, but by no means can it say with absolute accuracy if one is actually lying. polygraph operators commonly use a technique known as 'Stimulation', or 'stim' whereby they try to convince the subject that the polygraph will catch them out if they lie. one way some operators do this is by getting a deck of cards, asking the subject to select one and place it back in the deck. they then 'use' the polygraph to find out which card the subject selected - funny thing is, with this 'proof', it is not uncommon for the cards to actually be marked! now if even the polygraph operator needs to cheat and they are trained at reading the machine, how confident are you it can accurately catch a liar? especially if the subject suffers a degree of psychopathy or sociopathy, whereby even lying about a horrendous crime may not register a determinable output as there is no fear of being caught or no feeling that what they did was wrong, so they might be let off when indeed they did do the crime. it works both ways - the liar might get away with it, the innocent might be accused.
in truth, there actually is not very much scientific evidence of the polygraph and it's accuracy and I for one would not like to see it used to make or break someone's future regardless of the degree of their crime or charge.