How are lie detectors credible

Lie detectors and the truth

In Europe lie detectors are controversial, in the USA they are an everyday device in crime investigation - in the future it will be possible to look directly into the brain

Scientists call the detection of lies through the physical reaction of suspects forensic psychophysiology. Colloquially, on the other hand, we speak of lie detection. The picture is well known from American films: A suspected perpetrator sits wired in front of the investigator and after every answer, needles knock out of the paper. If they start to whiz back and forth like crazy, then the suspect is lying. This device is called a lie detector, more precisely it is called a polygraph. Electrodes attached to the body and other sensors measure the respondent's reactions, usually depth and rate of breath, blood pressure, cardiac activity, and perspiration.

If you lie, you get aroused and the resulting emotions show up physically - that is the basic assumption of lie detection. The measurement protocols are only part of the procedure, because the interviewer and his standardized, structured test questions supplement the mechanical recordings. The two most commonly used tests are, first, the Control Question Test, which mixes relevant questions about the crime with control questions. The reactions during the answers should then provide information about the credibility of the respondent or his guilt or innocence. The second is the guilty knowledge test, in which specific details of the crime are asked to see whether the suspect reacts differently to facts that only the guilty party can know than to credible but untrue allegations.

Use of lie detectors

In the USA in particular, polygraphs are used, both by the police to identify criminals and by the government in sensitive security areas (see American Polygraph Association). All American secret services use the polygraph for recruitment and regular security checks of employees. The last spectacular operation that became known to the public took place in the US nuclear weapons laboratory in Los Alamos when data media went missing there (tell me where the bombs are - where have they gone?).

In total, polygraphs are regularly used in more than 50 countries, some of them by companies, to check their personnel. In addition to the USA, this also includes Japan, Australia, South Africa, China and Israel. This is astonishing because the physiological signs of lies are still controversial among psychologists to this day. It is known that the devices have an error rate of at least 10 to 20 percent, and this number increases rapidly when inexperienced people conduct the tests as interviewers.

Again and again proponents of polygraphs claim that secret services are able to train their employees in such a way that they can systematically outsmart the lie detectors (perfect liars). The use of psychopaths is also extremely controversial, as they not only constantly manipulate others, but also lie notoriously without reacting particularly physically (the psychopaths are among us). The lack of a sense of guilt, which often occurs among perpetrators in the area of ​​sexual abuse, tends to lead to incorrect results (see Can you measure lies?).

However, the UK government now plans to put the long-cherished plans (bodies don't lie) into action and use polygraphs to monitor convicted sex offenders while on probation. The pilot phase has long since started and, according to the will of the government, the procedure should soon be carried out nationwide in certain regions - also by private companies.

No evidence in the Federal Republic

In Germany the lie detector is considered unreliable and is therefore not permitted as evidence. A study by the Institute for Prevention Research and Safety Management in Münster came to the clear conclusion in 2000:

The crime-preventive range of the investigation procedure with a polygraph tends towards zero, because the researchers in the present project, contrary to the opinion held worldwide, succeeded in arbitrarily misleading even experienced users of the polygraph into the desired direction of the statement without them noticing. This "deception process" is methodologically simple and in principle can be learned by anyone within a day. This means that the lie detector test fails as a truth-finding tool and in this respect does not develop any crime-preventive range. On the contrary, the compulsory application could have a criminogenic effect, because those practiced in deceptive behavior could "prove" their supposed innocence. These findings are likely to have an impact on the application of the method worldwide, but at least in Germany the previously practiced test method must be buried.

In criminal proceedings, the polygraph test is inadmissible as evidence, even though it does not violate constitutional principles such as human dignity if it is voluntarily taken, the Federal Court of Justice ruled in 1998 (judgment of December 17, 1998. Polygraph test as evidence). In 2002 the Bavarian Ministry of the Interior stopped an advance by the Munich police who had planned an attempt with lie detectors. Most recently, in 2003, the Federal Court of Justice decided to stop the practice in civil proceedings whereby potential abusers presented lie detector tests in their pain and suffering claims in order to prove their innocence.

New procedures

In the meantime, other methods for recognizing liars are also on the market and compete with the classic polygraph. Private individuals should also be able to use voice analysis to determine whether the conversation partner is telling the truth on the phone (caution: lie detector). Or a heat meter should determine whether a liar tends to turn red (new lie detector technology for counter-terrorism).

Attempts to look directly into the brains of suspects are more serious. For years the activity of the brain has been scanned more and more precisely by neuroscientists, although it is often still a matter of dispute what exactly they see on their beautifully colored images (cf. In Pepsi drinkers, it glows in the prefrontal cortex). To be able to look terrorists straight into the brain naturally sounds tempting. No wonder that the CIA has already invested in relevant research in the past (see A brain scan as a lie detector).

As the BBC recently reported, the US Department of Defense is now investing large sums in research by Jennifer Vendemia of the University of Carolina. She claims that her system for recognizing lies in the brain waves has a maximum error rate of six percent and is thus far superior to the classic polygraph tests. More than 120 electrodes are attached to the groin and scalp to make the brain waves immediately visible. Vendemia is certain that their procedure will prevail:

The polygraph test checks for changes that affect the peripheral nervous system - skin conductivity, breathing, heartbeat - as indicators of whether someone is telling the truth. Our research focuses on the central nervous system, which causes all of these peripheral responses. It is the difference between second-hand information and information straight from the source. (...) When someone decides to lie, he activates a certain area of ​​the brain. We could see the brain activity that occurs when someone starts to lie.

(Andrea Naica-Loebell)

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