Detecting Lies- page 2 | Science | Smithsonian

Detecting Lies

From chewing rice to scanning brains, the perfect lie detector remains elusive

(Continued from page 1)

The Guilty Knowledge Test

In the late 1950s, modern deception research took a new turn, when psychologist David Lykken of the University of Minnesota adapted polygraph interrogation with his guilty knowledge test.

A typical polygraph question asks a suspect whether he or she committed a crime. The guilty knowledge test focuses its questions on knowledge that only a perpetrator would have. Say, for example, you stole a purse from a woman wearing a bright green dress. A polygraph examiner might ask: "Did you steal the dress?" A good liar could control his response and pass the exam. Lykken would ask two questions: "Did you see a green dress?" and "Did you see a blue dress?" Regardless of your answer, the mere mention of the incriminating detail would cause a noticeable blip in your physiological reactions.

In 1959, Lykken published the first study showing the effects of this method. He had some 50 subjects enact one or two mock crimes, while others enacted none. Then he asked everyone to take a guilty knowledge test. Based on physiological responses, Lykken correctly categorized about 90 percent of the subjects, he reported in the Journal of Applied Psychology.

One of the subjects, it so happens, was a Hungarian refugee who had twice fooled the KGB about his anti-Soviet involvement. After a 30-minute interrogation, Lykken had identified which of the two mock crimes this subject had committed.

The P300

One day in 1983, the phone rang in J. Peter Rosenfeld's psychology lab at Northwestern University. It was a CIA agent. He wanted to know if Rosenfeld would run the agency's new lie detection program.

Rosenfeld froze. How did the CIA know he had planned to start researching deception? After all, he had only told a trusted colleague, and his mother. But it soon became clear that the agent had been calling several researchers in the hopes of luring one to direct the new program. Rosenfeld declined but recommended a promising graduate student, and for the next several months, broad-shouldered men in suits popped out from behind trees on Evanston's north campus.

Finally, the agency decided to hire the student. She flew to Washington, D.C. and took a polygraph test as standard job-screening procedure. But as her husband and children prepared for a new life, she failed the test on a question about her sexuality and lost the job, Rosenfeld says. "It was a simple case of the polygraph making a mistake, but the CIA has to be more safe than sorry," he says. "At that point, I said we might as well try to have one [a lie detector] that's based on science."

Rosenfeld settled on a method that combined Lykken's guilty knowledge test with brainwave research performed by Columbia University researcher Samuel Sutton. In the 1960s, Sutton had discovered that human brains show a burst of activity 300 milliseconds after a person sees a distinct image. Rosenfeld's premise was simple: If a woman wearing a green dress is robbed, then the perpetrator's mind will store an image of the dress, and his brain will respond a certain way when later confronted with this image.


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