No lie detector is 100 percent accurate, one reason their results are not admissible as evidence in many courts.
But Truster's makers say that their test is more accurate than the polygraph test, in which a range of a subject's physiological reactions are measured by hooking up the subject to complex apparatus. Because Truster does not involve such an invasive procedure, they say, it is less likely to cause the kind of stress that can affect the very reactions it is trying to gauge.
Truster's inventors say their approach, based on mathematics and psychology, is different. They concentrated exclusively on level of stress in the voice by measuring its "cognitive messages" - very-low-frequency waves that are not audible.
"It's only the beginning of our research," says Shlomo Bruck, a polygraph expert who is serving as an independent research consultant to Makh-Shevet, which makes Truster. Mr. Bruck is head of Uran Polygraph, a private polygraph firm in Tel Aviv, and former head of the polygraph division of the national police in Israel.
"I think the polygraph is still more accurate, but you can use the Truster for different things. You can use it when a large group of people is suspect, and you can talk to people through the phone, or for screening job applicants," he says.
When someone's lying, subtle changes in the vocal cords are said to occur as a result of the stress, producing a distorted sound wave. Using a complex algorithm and nine different parameters, the inventors say they are able to pinpoint whether the person's stress is caused by lying, excitement, exaggeration, or an emotional conflict.
To test the product, the developers tried it on tapes from Israeli intelligence institutions where the interview subjects are known to have lied. With 85 percent accuracy, Truster was able to determine when they were lying.
Truster may tell the user that the "Subject is not sure" when the person could be feeling unsure about choice of grammar or verb tenses. And the interviewee must also be speaking in his or her native tongue. Except for people who are thoroughly bilingual, much of the hesitation registered by the computer will actually be part of the translation process.