T is a time-honored interrogation tool and a staple of film noir: the lie-detector test that can incriminate or exonerate.

But such tests need not involve strapping someone to a machine. In fact, they may not require the subject's presence - or awareness - at all. And their use is growing far beyond criminal investigations.

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Increasingly, lie-detector tests use voice stress analysis, a technology that has been around for decades but that has gained in popularity as the software at its heart continues to be refined.

"It can really be done anywhere," said Detective Pat Kemper of the Springfield Township Police Department in Ohio, who says he has used the voice-based testing to question thousands of suspects over the last decade. "It can be done via a telephone recording. It can be done covertly. You can use it for anything."

Indeed, beyond its applications in law enforcement, proponents of the voice-based technology see its utility in everything from telemarketing to matchmaking. In Britain, a growing number of insurance companies have been using it to screen telephone claims in hopes of rooting out fraud - a goal they say has been borne out, both in fraud detection and in deterrence. One insurer, Admiral, says 25 percent of its car-theft claims have been withdrawn since it began using the system a year ago.

But the technology's reliability is still a matter of debate, and its migration from the interrogation room to the call center has raised concerns about potential privacy implications. Voice analysis of this nature, after all, can easily be conducted without the speaker's knowledge. Now that it is being used in the insurance industry, for example, the concerns include how a suspect claim might affect a customer's subsequent applications for insurance.

The makers of one system, known as the Computer Voice Stress Analyzer, try to address such worries by controlling access to the technology. "We only sell to law enforcement and the government," said David Hughes, executive director of the National Institute for Truth Verification, the company selling the system.

The analyzer has been available since 1988, and the company says it is used by over 1,400 law enforcement agencies across the United States, as well as by other state and federal agencies including the Defense Department. "In private industry, you're doing it for profit, so there's always a concern about ethics violations,'' Mr. Hughes said. "You can't be cavalier."

But that distinction does not appease everyone. "Government is not necessarily more responsible than private industry," said Bob Barr, a former congressman from Georgia who is a privacy consultant for several organizations. "The government can put you in jail if it doesn't like what it finds."

The several available applications of the technology work on the same basic principle: that the human voice contains telltale signals that betray a speaker's emotional state, like the intent to deceive. By analyzing small, often inaudible changes in the voice and visually displaying them on a computer screen, the techniques are thought to recognize not only veracity, but also a gamut of emotions ranging from anxiety to arousal.

With the Computer Voice Stress Analyzer, for example, subjects are asked several control questions, like the color of the walls, to gauge normal vocal response levels. During the questioning, the interviewer sees the computer's interpretation of the responses immediately, although deciphering the charts generated by the computer requires training. Interviews or phone conversations can also be recorded for later analysis.

David Watson, chief operating officer of V Worldwide, which distributes a voice stress technology in the United States, said his company was sensitive to the potential privacy concerns, and he emphasized that the technology should be only one part of a broader screening process.

"We don't advocate that people make decisions based on this information," he said. He added that phone-based screening was less intrusive than a traditional polygraph test, which measures responses like perspiration and blood pressure, and that it "eliminates a lot of the racial and ethnic profiling that goes on either consciously or subconsciously" in face-to-face interviews.

Mr. Watson said government and law enforcement agencies in Arkansas, Florida, Texas, Georgia and New Mexico were using or testing the product. He also said it was being tested by several American insurance companies, which he did not identify. A spokesman for the American Insurance Association, a trade group, said he knew of no plans in the industry to use the technology.

Nemesysco, the Israeli company behind the phone-based technology marketed by V, has been developing its "layered voice analysis" techniques since 1997, initially in the form of consumer-level gadgets known as the Truster and Love Detector.

Its founder, Amir Liberman, laments that his technology is often misunderstood. "There is no such thing as a lie detector," he said. "A lie is not even an emotion."

Mr. Liberman noted that there are many reasons for lying: for personal gain, to avoid harm, to protect someone's feelings or to avoid shame or embarrassment. Each, he said, has its own psychological motivations and thus its own physiological manifestations. They are all various kinds of "self-defense mechanisms of the body," he said, "coming from a completely different place."

He said his company has tested "tens of thousands" of subjects over the years - for example, by showing them cards with the word "red" written in blue, and then asking them what color they saw. This is likely to create confusion, he said, and the resulting vocal sample is then compared with the same subject's previous samples "to identify the cognitive stress." Subjects are also asked about ex-spouses and coming weddings, "things that we know have very intensive emotion," and the vocal responses are then correlated to various emotional states.

Mr. Liberman said that improvements in the technology accounted for part of the increased acceptance, but that public perception had also been crucial. He cited the microwave oven, which took decades to find routine use. "That was also very frightening at first, nuking the food," he said.

But there is no consensus that the concept behind voice stress analysis is sound. "The people fomenting this nonsense" come and go, said David T. Lykken, an emeritus professor of psychology at the University of Minnesota who has been studying and publishing on lie detection for over 50 years. "It is implausible to imagine that the only times that your body shows increased arousal when you answer a question is when you're being deceptive."

Studies by institutions including Washington University and the Justice Department have yielded negative or inconclusive findings. A 2002 investigation by the Air Force Research Laboratory in Rome, N.Y., for example, said that voice-based lie detectors "do recognize stress through voice analysis; however, although these systems state they detect deception, it was not proven."

But the National Institute for Stress Verification, which sells the Computer Voice Stress Analyzer, posts links at its Web site (www.cvsa1.com/studies.php) to a range of studies that the company sees as bolstering the technology. And Mr. Hughes, the company's executive director, said the results in the field spoke for themselves. "You can talk about these scientific studies by people who don't know what they're doing, but when you have 1,400 police departments with over 5,000 examiners using it, there's got to be something going for it," he said, citing United States sales figures.

The very presence of any such technology may weed out faint-hearted fraud artists unprepared to tussle with the computer. "We have a lot of people just hanging up then and there" when told about the phone-based detector, said Lior Koskas, director of business development for Digilog, which distributes the Nemesysco technology in Britain to Admiral and other customers. "A very nice percent of the total amount of claims just go away on their own."

Lieut. Christopher Calabrese of the Westchester County Police Department in New York, who said he has been using the Computer Voice Stress Analyzer system for about seven years, is familiar with this deterrent effect. "Sometimes people are more apt to give you information looking at an instrument that they believe is going to tell the truth anyway," he said.

But he believes the technology is solid. "It's a tool to give us direction in which way to go," he said. "If we have 25 possible suspects in a homicide, and we clear all of them except for one or two, then we will focus on those two."

Mr. Hughes also rejects contentions that voice analyzers are mere props to scare suspects into confessing. Given the devices' cost, which can exceed $10,000, he said, "you think they're working on a placebo?"

The legal admissibility of voice-based lie detection in the United States remains limited, much as it has for the polygraph. The voice machines are permitted in employment screening for government jobs and private security companies, in which case subjects must be made aware that the devices are in use. Although no states admit voice stress analysis as court evidence, several allow its use in criminal investigations, sometimes covertly. And the Aviation and Transportation Security Act of 2001 cites voice stress analysis as a potential tool in airline security.

While academic and legal experts debate the merits of the technology, its developers continue to find new uses for it. V Worldwide is working on a system that lets airport screeners apply a 30-second, five-question test to determine whether a passenger should be put on a fast track or interviewed further. And Nemesysco has plans for its algorithms that include analyzing politicians' speeches, assessing post-traumatic stress disorder and even helping with affairs of the heart: it recently introduced Mad Love, "a cellular service which allows you to find out if the person you're speaking with is attracted to you," Mr. Liberman said.

What about letting potential customers use voice stress analysis on insurance salesmen? "It's a good idea," Mr. Koskas of Digilog said. "But I don't think it's ever going to materialize."

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