This article first appeared in Issue 8 of our free digital magazine CURIOUS. 

We've all seen lie detector tests, be it in cop movies, crime procedurals, or The Simpsons. Generally speaking, the “bad guy is hooked up to the polygraph test and informed that if they lie, the people conducting the test will know. A few questions later and the police have either eliminated a sweating suspect or doubled down on forcing a confession based on the results of a pen drawing wild zigzags on a moving chart.  

But how accurate are they really? Well, not at all. 

First of all, it’s important to note polygraph tests, or lie detectors as they are more commonly known, don’t actually detect lies. They measure psychophysiological bodily responses that may indicate anxiety, nervousness, or fear that can be interpreted as inferring deception based on a set of – and it’s very important to note here – not standardized psychological questions. So the machine is measuring a physiological response, but what that means is entirely down to human interpretation. 

How do polygraph tests work?  

Polygraphs monitor three things: your breathing, blood pressure/heart rate, and skin conductivity (i.e. sweat). A pneumography around your chest measures your breathing, your heart rate is monitored with a blood pressure cuff, and electrodes attached to your fingertips measure sweating. 

During an examination, you will be asked control questions to assess your baseline physiological responses. These are a mixture of “probable lie” control questions about yourself and past deceptions. They are based on the idea most people have lied about jaywalking or some such misdemeanor but want to come across as honest so tell a “white” lie, and “accusatory” relevant questions – for example, in a robbery case, asking if you’ve ever stolen anything. The polygraph measures how you react using pens on a moving chart that shows blips if your heart rate quickens or you begin to sweat. If you caused a blip more often during the probable-lie question, you pass, if you reacted more during the relevant questions, you fail.  

Of course, the problems with this are immediately obvious. There is no scientific evidence that any pattern or group of physiological responses are unique to lying. A beating heart and sweaty hands may indicate you are dreading getting caught – or it could mean you are wildly attracted to the person conducting the test since both physiological responses are common in arousal too. Inferring meaning behind a bodily response is hard and there is no one-size-fits-all.  

Evidence in support of the lie detector is scant  

"Almost a century of research in scientific psychology and physiology provides little basis for the expectation that a polygraph test could have extremely high accuracy," a review by the National Academies of Sciences, Engineering, and Medicine explained in 2003.  

"Polygraph research has not developed and tested theories of the underlying factors that produce the observed responses.”  

So, you may have a physiological response to a lie detector test, but then being in police custody accused of something you may not have done can also do that to a person. Basically, it doesn’t address the placebo effect that may be in play. Polygraph tests are not assessing your truthfulness but your fear, which could be all over the place for several reasons. 

An honest person may be nervous despite answering truthfully and a dishonest person may not be anxious because they may actually be a sociopath, who are generally good at hiding the fact they are a sociopath.   

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“There is evidence suggesting that truthful members of socially stigmatized groups and truthful examinees who are believed to be guilty or believed to have a high likelihood of being guilty may show emotional and physiological responses in polygraph test situations that mimic the responses that are expected of deceptive individuals,” the review authors write. 

Examples of this include ethnic minorities facing structural racism, for example, in the US a three-year analysis from 2015 to 2018 found that in 800 jurisdictions across the US, Black people were five times more likely to be arrested than white people for the same perceived crime. If you go into the polygraph test knowing the system is already against you and you’ll have to work much harder than your white counterpart to be considered innocent until proven guilty, you’re going to sweat.   

Would you be given one? 

Most psychologists and scientists consider polygraph tests controversial as there is little good research to validate their usefulness and, as we will demonstrate soon, they are easy to beat so are not deemed accurate. And yet, such is the way of the world, it's possible you may have to do one anyway. Despite the lack of evidence to support them, the tests are still used by law enforcement and by employers (e.g. police, fire departments) to vet candidates in the US and the UK to name a few.  

You cannot be forced to take a polygraph test unless you are under the supervision of a court. However, whether polygraph tests are admissible in court depends on your location. In the US, for example, the Supreme Court helpfully left it up to individual jurisdictions to decide whether to allow them as evidence. 

Fortunately, as the polygraph test is not measuring truth, merely a few bodily responses which the people conducting the test then interpret, the trick to beating the lie detector test is to alter your body reactions at key times. This sounds harder than it is, and with a few well-timed sweats or anal clenches, you'll likely mess up your test enough to make the results meaningless for anyone trying to glean anything from it.  

How to beat the test 

Your first goal is to disrupt the test by providing unusual baseline responses or unusual readings in the initial control question portion before the main part of the test. As we mentioned, if you have more blips in this section than the “relevant” questions you are deemed more honest. 

According to the authors of The Lie Behind The Lie Detector, George Maschke and Gino Scalabrini, you should quicken your breathing during control questions, then breathe normally during the main questions, and relax by thinking calm thoughts before answering. 

During the probable-lie control questions, they suggest you should also lie and bite hard on your tongue to set off other physiological reactions and throw off your control readings. In the movie Ocean’s Thirteen, for example, one character hides a thumb tack in their shoe which they then stamp on at key moments to produce these responses. However, these days, you may get asked to remove your shoes.  

In order to produce sweat, you can think of scary things during the control questions, or alternatively, do tough math problems in your head. But what about pulse and blood pressure? For that, you can turn to our good old reliable friend the butthole.  

This odd tip comes from former Oklahoma police officer Doug Williams, who used to perform polygraph tests as part of his career, before becoming skeptical of the technology and publicly denouncing it. He could control his breathing when using the machine, but couldn't figure out how to control his pulse, sweat, and blood pressure. 

"It wasn't until my friend came in and started talking about the pucker factor and tightening up the anal sphincter muscle when he was under stress," he told This American Life. 

"After he left, I just hooked myself up to the polygraph test...and tightened up my anal sphincter muscle like I was trying to stop my bowel movements and low and behold there was the most gigantic, wonderful, naturally occurring cardio rise, accompanying a [galvanic skin response] rise." 

During the actual test questions is your time to relax and focus on your breathing patterns. Even if you do have a slight response to the relevant questions, your work in the control question section should have already produced a much stronger response.  

"Your mind should be more at ease knowing that you and not your polygrapher are in control," Maschke and Scalabrini write. 

CURIOUS magazine is a digital magazine from IFLScience featuring interviews, experts, deep dives, fun facts, news, book excerpts, and much more. Issue 11 is out now.

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