You see them on TV shows and police use them in interrogations, but "lie detector" tests aren't even admissible in most courts. Why are they even used? And should you ever take one if you're suspected of a crime?
What is a lie detector test?
Lie detectors (the common term for polygraph machines) take a supposedly scientific approach toward the detection of lies by measuring several physiological indicators of stress.
Subjects are hooked up to a variety of tools that measure changes in their blood pressure, pulse, breathing and skin conductivity while they're asked a series of questions. The idea is that deceptive answers can be shown through involuntary changes in a person's physiological responses.
Why are they controversial?
According to the American Polygraph Association, modern polygraph machines are close to 100% accurate. According to critics, however, the best polygraph machine around isn't much better at detecting the truth than flipping a quarter: the false positive rate is around 50%. That means that a totally innocent person has a 1 out of 2 chance of being accused of lying.
In fact, if you're a habitual liar or a career criminal, you might stand a better chance at passing a lie detector than you would if you are innocent. Someone who is adept at lying or calm under pressure is going to show less of a physiological response than someone who is scared and nervous because they've never been involved in a criminal investigation before.
Additionally, the physiological factors being measured can all be affected by a variety of drugs, including things like diazepam, which help suppress people's emotions.
Why do the police use them?
The police use lie detector tests all the time as an investigative tool. Generally, they offer them to people under suspicion of a crime as a way to "prove" that they aren't guilty. People accept, thinking that the machine will show their innocence and they can then move on with their lives.
In reality, the polygraph is used as a tool to make already nervous suspects frightened enough to confess. If you flunk the lie detector test, the "evidence" of your guilt is presented to you while you're pressured to make a confession. The pressure can be enough to make even innocent people confess - false confessions are the third leading cause of wrongful convictions in this country.
Should you take one?
Generally speaking, there's usually no good reason to take a polygraph. Most likely, the police wouldn't be asking you to take one if they had enough evidence of your guilt to charge you without it. If you're innocent, you don't want your nerves to cause you to fail the test and increase the focus in your direction.
Even if you pass a lie detector, there's no guarantee that the police will stop looking at you as a suspect. If you are eventually charged with a crime, it can't help you in court. Since 1998, attorneys haven't been able to present polygraph evidence in court as a way of proving their client's innocence.
If you're considering taking a lie detector test, talk to your attorney first before you agree to the examination. All situations are unique, and your attorney will be able to advise you best. For more information, contact a legal office like Baudler, Maus, Forman, Kritzer & Wagner, LLP.