Moreover, the theory holds that deception produces distinctive physiological changes that characterize lying and only lying. This notion has no empirical support. Undeniably, when being dishonest, people can feel great turmoil and a polygraph can measure this turmoil. Some control questions may make a person appear guilty. Such questions may force a subject into a minor lie or ask about an invented crime that nonetheless makes the subject nervous.
Lie detectors are especially unreliable for truthful people.
Those who run a special risk include people who get upset if someone accuses them of something they didn't do, people with short tempers, people who tend to feel guilty anyway, and people not accustomed to having their word questioned. All of these feelings can change heart rate, breathing, and perspiration and their heightened feelings are easily confused with guilt. It has also been shown that polygraphs are easily manipulated. Four hundred milligrams of the tranquilizer meprobamate taken an hour or two before a polygraph session can make it virtually impossible to spot a liar by his physiological responses. However, do not try sighing, coughing, or clenching your fist or arm.