He committed a crime but acted as if nothing had happened. Can a lie detector expose his excellent acting skills?

Tang Yicheng

In the fairy tale, Pinocchio's nose would grow an inch when he lied. Although it is a fairy tale, science has proven that people do undergo some physiological changes when they lie. In criminal investigation, it is a common method to use polygraph technology to assist in investigation. When it comes to polygraph, we can't help but have many questions. For example: How does polygraph technology work? Can someone fool a polygraph? Will polygraph technology wrongly accuse a good person? Today, let's find out.

In fact, as early as 1000 BC, the wise ancients invented many "rustic" lie detection techniques. For example, the Indian chewing rice method. They observed such a phenomenon - fear will lead to a decrease in the amount of saliva secretion of an individual, and based on this, they designed a "lie" test method in which the suspect chews dry rice and then spits it out. There is also the Indian holy monkey method. Legend has it that if a liar touches the tail of the holy monkey, it will cry; while if an honest person touches it again and again, the holy monkey will be quiet. In fact, this is to use the principles of psychology to "catch" the liar. When trying a case, the judge puts a monkey with carbon powder on its tail in a dark room and asks the suspects to enter the room one by one to touch the monkey's tail. The liar dares not touch it out of guilt, while the honest person touches it calmly. As a result, the person with carbon powder on his hand is naturally not suspected.

Although the above lie detection methods have some rationality, they are generally absurd in the eyes of modern people. The invention of the polygraph is a sign that lie detection technology has bid farewell to the "earthy" and moved towards scientificization. You may not know that the modern polygraph was invented by William Moulton Marston, the creator of the comic character "Wonder Woman". Friends who have seen Wonder Woman will be impressed by the "Lasso of Truth" - when the bad guy is trapped by this golden lasso, he will tell the truth of the incident. Of course, the polygraph invented by Marston is not as magical as in the comics. It is actually a set of electrode systems that monitor autonomic nervous system reactions.

The most common polygraph contains records of three physiological signals: the breathing signal is usually displayed at the top of the screen, recorded by the chest strap and abdominal strap respectively, the waveform of the skin electrical response is displayed in the middle of the screen, and the waveform of the pulse signal is displayed at the bottom of the screen.

When using a polygraph, the interval time setting from the end of answering the previous question to the beginning of asking the next question can give the sympathetic and parasympathetic nervous systems sufficient reaction time, so that a more obvious reaction waveform can be displayed on the screen of the polygraph, and the target object being tested can only answer "yes" or "no" when answering questions. When the human body is in different emotional states, the pulse and breathing will change significantly. When a person lies, the most distracting thought in his heart is the worry that his lies will be discovered, and the changes in pulse and breathing are important indicators. In addition to the changes in pulse and breathing, the skin electricity, known as the "third kind of contact", is also a recording indicator of the polygraph. This indicator was first discovered by Ferry and Taczanov, and it shows the changes in the skin's electrical conduction when the body is stimulated. Sweat glands are the place where human sweat is produced. When the human body is in an excited or nervous state, the body temperature will rise, psychological activities will intensify, and the sweat glands will secrete sweat. Under the action of sweat, the skin's electrical conductivity will change, which is the basis of skin electricity lie detection. For example, when a person lies, he becomes nervous, which causes more sweat. At this time, the skin resistance decreases, the skin current will increase, and the amplitude of the skin electricity will increase. This hidden "little tension" will be discovered by the sensitive lie detector.

A Thousand Lies: The Embarrassment of Polygraphs

However, with the popularization of polygraph technology, some countermeasures against polygraphs have also emerged.

Taking fMRI lie detection technology as an example, some subjects will deliberately cause huge changes in fMRI signals by shaking their heads. This change will cause obvious signal noise. If these things are done at different times during the scan, it is enough to make the data worthless. In addition, a study conducted by George Ganis and his colleagues in 2011 showed that although they could detect the subjects' birth dates very accurately, if the subjects used a simple countermeasure when they saw dates other than their birthdays - imagining a specific finger movement (stimulating corresponding brain activity), the accuracy rate would drop to 33%. More worryingly, some "natural liars" are immune to polygraphs. Such people are called "psychopaths". They have no sense of shame for crimes and no sympathy for others. The more violent the event, the more excited they are. What's more terrifying is that psychopaths have abnormalities in the prefrontal and limbic regions (especially the brain regions that control fear responses). The amygdala responsible for predicting pain will not normally send the necessary fear signals to the central nervous system, which means that they will not feel nervous, worried, and other emotions when lying, and naturally there will be no detectable physiological signals. Therefore, although many serious crimes are committed by psychopaths, it is particularly difficult to detect polygraphs on them in criminal investigations. Their character flaws have become their best weapon against polygraphs.

A future without lies: practical lie detection

In order to break through the bottleneck of lie detection technology, polygraph researchers have invented more lie detection technologies. For example: interrogators ask the subjects to do another task at the same time to increase the cognitive load (Imposing Cognitive Load, ICL) of the subjects. Generally speaking, liars have a higher cognitive load than ordinary people because they need to make up plots. In this case, continuing to increase cognitive load can easily make the subjects lose their composure. Dr. Gil Luria and Dr. Sara Rosenblum, researchers at the University of Haifa in Israel, asked test volunteers to write two paragraphs of text on an electronic touch screen with an electronic pen with a pressure sensor tip, one of which was true and the other was fabricated. When the volunteers wrote the text, the researchers monitored the pressure of their pen holding, the length of the strokes, the height and width of the letters, and the time they lifted the pen off the touch screen when writing. The results showed that when the testers lied, they put a little more force on the electronic touch screen, and there were also obvious differences in the length of the strokes and the height and width of the letters. Compared with other methods, handwriting tests will not arouse the test subject's defensiveness and are more likely to capture real psychological activities. At the same time, handwriting tests provide various indicator standards in the test process, and objective judgments can be made based on these standard values, thereby effectively avoiding test bias caused by people's subjective assumptions.

Although lie detection technology has made great progress and has been supported by technology, there is still no so-called "perfect" lie detection equipment, and the struggle between lies and lie detection will continue. We have reason to believe that technology that makes lies impossible to hide will eventually appear in our lives. After all, technology originated from the beauty of human nature, and it should also create a better future for mankind, right?

Reviewer: Mao Lihua, Associate Professor, School of Psychology and Cognitive Science, Peking University

This article is produced by Science Popularization China-Starry Sky Project (Creation and Cultivation). Please indicate the source when reprinting.