Lie detector tests—or polygraph tests, in more scientific terms—are rarely used in criminal trials. The theory underlying a lie detector test is that lying is stressful, and that this stress can be measured and recorded on a polygraph machine.
Lie detectors are called polygraphs because the test consists of simultaneously monitoring several of the suspect's physiological functions—breathing, pulse, and galvanic skin response—and printing out the results on graph paper. The printout shows exactly when, during the questioning period, the biologic responses occurred. If the period of greatest biologic reaction lines up with the key questions on the graph paper—the questions that would implicate the person as being involved with the crime—stress is presumed. And along with this presumption of stress comes a second presumption: that the stress indicates a lie.
Supporters of lie detector tests claim that the test is reliable because:
On the other hand, critics of polygraph testing argue that:
The courts in most jurisdictions doubt the reliability of lie detector tests and refuse to admit the results into evidence. Some states do admit the results of polygraph tests at trial if the prosecution and defendant agree prior to the test that its results will be admissible.
You may wonder how, in the absence of a confession, a polygraph operator can confidently determine whether a person is lying. Don't most people—even innocent ones—get stressed when being asked questions that might land them in big trouble? Yes, but the polygraph operator has techniques to overcome this problem.
Before getting to the nitty-gritty of the issue ("Did you do it? Were you there? Do you have any personal knowledge of what happened?") the operator first asks a series of questions, some of which are emotionally neutral and some of which are calculated to cause emotional discomfort based on the test subject's personal circumstances.
The subject's physiological responses to these questions are meticulously calibrated. Then, when the operator gets around to the core questions, the responses to those questions can be measured and compared to the responses produced by the neutral and control questions. It's true, a lot of devils can live in the space of relative anxiety measurements, but numerous independent tests have indicated an accuracy rate in the 80-90% range.
So if these tests are so accurate, why aren't they required in every situation where truth is at issue? Why spend billions of dollars on jury trials and independent prosecutors when we could just "wire up" the key witnesses and get at the truth with no fuss and no muss?
Some people are so divorced from morality or a guilty conscience that they may test honest—because they are really good liars or have convinced themselves of a truth that isn't the truth at all.
It may also be possible to change the results through other methods, such as yogic or biofeedback training, the "nail in the shoe" trick (putting a sharp nail in your shoe to cause yourself pain during the questions to skew the polygraph results), or using a legal or illegal drug to calibrate one's emotions. Simply put, there is no way to turn a lie detector test into a slam dunk. For this reason, most courts will not admit polygraph results unless both sides of the case agree.
To learn about all stages of a criminal case, get The Criminal Law Handbook: Know Your Rights, Survive the System, by Paul Bergman and Sara J. Berman (Nolo).