Wouldn’t it be nice if there were a machine that could tell you when someone is lying?
Some people believe that there is. It’s called a “polygraph”–popularly known as a “lie detector”–and it’s been in the news lately, both in Washington and in Regina. Other people, however, will tell you that the polygraph is a fraud, no better at telling who’s lying and who isn’t than flipping a coin.
Lie detectors are based on the theory that the stress of being caught lying causes changes in bodily functions. The ancient Chinese understood this. They’d ask a subject an easy question, such as, “Is your name Li?” Then they’d ask the subject to put a handful of rice, whose grains they had counted, into his mouth for three seconds, then spit it out. Then they’d ask a tough question, such as, “Did you steal your neighbor’s pig?,” and ask him to hold a handful of rice in his mouth again for three seconds and spit it out. If more grains came out the second time, they’d figure he was lying because the stress made his mouth drier, which meant fewer grains stuck in it.
The polygraph (“poly” means multiple, and “graph” means writing–the device makes marks on a strip of moving paper), introduced in the 1920s, doesn’t use rice; instead, sensors record the breathing rate, pulse, blood pressure and perspiration. Breathing is measured with sensors that detect the movement of the chest and lower abdomen; pulse and blood pressure are measured with a standard blood-pressure cuff; and electrodes that measure how well the skin’s surface conducts electricity reveal the perspiration level. Some polygraphs also measure arm and leg movement and blood supply to the skin.
The procedures surrounding a polygraph test are strict. First, the person has to agree to take the test; being tested under duress could create stress that could skew the results. Second (somewhat surprisingly), the subject is told the questions in advance, again because a surprise question could produce stress whether the subject tells the truth or not. Knowing in advance that a question that will test truthfulness is coming, on the other hand, generates exactly the kind of stress the polygraph is intended to measure.
Easy questions the subject won’t lie about–“Is your name Bob?”–are asked first. The subject sees the polygraph spitting out its paper tape, which again creates stress as truth-testing questions approach, as the subject wonders how the lines on that tape will change. The easy questions also give the polygraph operator a baseline to work from. If subsequent questions cause the traces to change significantly in any way, the operator suspects the subject is lying.
Supporters of polygraphs–such as, for example, the American Polygraph Association (www.polygraph.org) claim studies show the polygraph is accurate around 90 percent of the time. Even the APA admits that a valid polygraph examination requires a properly trained examiner, a proper polygraph and the proper administration of the accepted testing procedure. That’s one reason why polygraph readings are, in many places (though not all), inadmissible as evidence in court.
For critics of the polygraph (www.antipolygraph.org), the fact that polygraph examinations are subject to so many variables makes them worse than useless. “20th century witchcraft” is one term I ran across.
The unreliability of polygraphs has made agencies like the U.S. Department of Defense, who would love to have an accurate lie detector to help it weed out spies, anxious to find new devices to replace it.
It may, for example, be possible to measure the presumed fear of detection directly. We know that fear is associated with a brain region called the amygdala. Researchers plan to conduct tests in which someone undergoing an MRI scan will be asked to lie. They expect to see not only activation of the amygdala–due to the fear of detection–but also changes to the rest of the brain’s activity, because lying requires a lot more extra thought than telling the truth.
Other research is focusing on the use of an infrared camera to detect the temperature changes caused by variations in facial blood flow during lying; the use of lasers to pick up muscular, circulatory and other bodily changes; the analysis of voice stress; and the monitoring of brain waves.
None of these technologies were available 10 years ago. If they pan out, the problematic polygraph may soon be extinct–but the detection of liars easier and more accurate than ever.
