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You're watching TV, you're 45 minutes into a tense crime drama, and the cops have their

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primary suspect wired up to a lie detector machine.

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Finally the truth, or maybe not. In fiction and on the news, polygraph tests are often depicted as basically infallible,

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unless you're a super genius serial killer with the ability to control your heartbeat

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in every drop of sweat. But in real life, there are plenty of cases where relatively normal, non-serial killers

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have beaten the test. On top of that, the polygraph isn't even measuring lies strictly speaking.

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Lie detector is an effective, but actually misleading, little bit of branding.

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What a polygraph test actually measures are a number of physiological responses that typically

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correlate with stress, respiration, perspiration, heart-rateration, heart rate, and blood pressure.

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Now lying is typically stressful, even when we don't feel guilty because deception takes

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effort and there are consequences to getting caught. To take all those readings, a polygraph machine has several key components.

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First is the pneumograph, a device that measures a subject's breathing.

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There are two major variants of pneumograph. The first uses two sets of rubber tubes, one that circles the chest and one that circles

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the abdomen. These loops stretch when the subject breathes, thereby recording the rate of respiration.

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The second type of pneumograph uses electrical impedance to measure breathing.

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So the test administrator places two pairs of electrodes on either side of the chest.

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Because nipples never lie, I guess? They then run a high-frequency, low-amplitude current through the subject's chest cavity.

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And because air is a better electrical insulator than human flesh, the chest blocks more of

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the current as the lungs expand. To measure how sweaty the subject gets when getting grilled, a mouse is placed on a desk

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in front of them and they game. No, I'm just kidding. A galvanograph is attached to their fingertips.

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This uses electrical current to measure the conductivity of the skin, which increases

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when damp. Next, there's the cardiospygmograph, which uses an inflatable blood pressure cuff to

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amplify the sound of the subject's blood flow, and therefore heartbeat, recording blood

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pressure and heart rate. Finally, there's the data acquisition system, or DAS, which collects data from each of these

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subsystems and converts it into a readable form. A bunch of squiggly lines on a paper readout, or, well, more likely these days, a screen.

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Modern DAS can even assist administrators through simultaneous computer analysis of

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the results. Now, that's a lot of delicious, quantifiable data that's being collected, but does it

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really mean that we can measure lies and solve crime?

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One of the most difficult aspects of applied science is just how many potential confounding

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variables there are in the real world, with the messiest being the human element.

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Polygraphs measure breathing, sweating, heart rate, and blood pressure.

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All physiological symptoms of stress, but there are tons of causes of stress that have

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nothing to do with lying to the authorities because you just murdered your spouse.

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For instance, having the authorities accuse you of murdering your spouse might be incredibly

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stressful. For that matter, so is jogging, drinking coffee, watching a scary movie, performing on stage,

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falling down the stairs, getting married, or being punched.

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And the human body doesn't really differentiate much between these various sources of stress.

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Any of them can turn you into a sweaty, stuttering mess if they're intense enough.

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For blood pressure readings, there is a known problem called white coat hypertension, where

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some patients have consistently higher blood pressure readings whenever they're tested

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in a medical setting like a hospital or doctor's office compared to when they're tested at

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home or with those automatic tests at the grocery store.

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This is typically because the patient finds going to the doctor stressful.

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Many innocent people then, even those who have never had a bad interaction with the

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police, find interacting with law enforcement scary, and those people are more likely to

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fail a polygraph. Another problem is that polygraph interpretation is, well, interpretation.

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No two people respond exactly the same to all stressors.

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And there's no hard line between pass and fail. Rather, administrators typically ask a series of irrelevant but uncomfortable questions to

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establish a baseline, and then they compare that to later, more loaded questions about

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the actual subject of the investigation. But this leaves a major opening for subversion.

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Others who know how the test works may intentionally speed up their breathing during the baseline

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questions or may slow their breathing during relevant questions.

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Some may take anti-anxiety medication or drink alcohol before the test to mask their stress,

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while others may simply walk in confident and retain their cool.

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Aldrich Ames, a chronic alcoholic and a spy for the Soviet Union, was given two polygraphs

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during his time at the CIA and passed both of them.

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The key advice given to him by his KGB handlers? Just relax, comrade.

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A major assumption of the test is that these reactions are involuntary, and to an extent

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they are. But almost everyone outside of the iron lung can control their breathing at least.

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And then from there, it takes a trivial amount of practice to intentionally speed up or slow

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down your heart rate, which also affects your blood pressure.

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In some ways, the innocent are the least prepared to take a polygraph test as habitual

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liars are less likely to show signs of stress when lying.

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What this all means, then, is that polygraph tests aren't terribly reliable, better than

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chance perhaps, but seriously flawed. There's no firm scientific basis for using these tools to determine guilt or innocence,

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and courts in most jurisdictions have ruled them inadmissible as evidence.

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That hasn't stopped US law enforcement from using polygraphs as part of their hiring process,

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though, nor has it stopped some researchers from trying to find a new 21st century equivalent

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to the polygraph, either through MRI scans of the brain, micro-expression tracking electrodes,

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eye tracking cameras, or even facial recognition technologies that's adapted to spot a lie

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at a distance via security cameras. It remains to be seen, though, whether these new lie detectors will be any more accurate

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than the old ones. If you enjoyed this video, do a polygraph.

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Prove it. No seriously, hit the like button, and if you didn't, you can dislike.

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And if you have any feedback for us, leave a comment, maybe one with a favorite outlandish

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lie. Here's one from us. My heart beats twice a minute like a blue whale, and I haven't blinked once this entire

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episode. If you don't believe me, go back and check.