You're watching TV, you're 45 minutes into a tense crime drama, and the cops have their
primary suspect wired up to a lie detector machine.
Finally the truth, or maybe not. In fiction and on the news, polygraph tests are often depicted as basically infallible,
unless you're a super genius serial killer with the ability to control your heartbeat
in every drop of sweat. But in real life, there are plenty of cases where relatively normal, non-serial killers
have beaten the test. On top of that, the polygraph isn't even measuring lies strictly speaking.
Lie detector is an effective, but actually misleading, little bit of branding.
What a polygraph test actually measures are a number of physiological responses that typically
correlate with stress, respiration, perspiration, heart-rateration, heart rate, and blood pressure.
Now lying is typically stressful, even when we don't feel guilty because deception takes
effort and there are consequences to getting caught. To take all those readings, a polygraph machine has several key components.
First is the pneumograph, a device that measures a subject's breathing.
There are two major variants of pneumograph. The first uses two sets of rubber tubes, one that circles the chest and one that circles
the abdomen. These loops stretch when the subject breathes, thereby recording the rate of respiration.
The second type of pneumograph uses electrical impedance to measure breathing.
So the test administrator places two pairs of electrodes on either side of the chest.
Because nipples never lie, I guess? They then run a high-frequency, low-amplitude current through the subject's chest cavity.
And because air is a better electrical insulator than human flesh, the chest blocks more of
the current as the lungs expand. To measure how sweaty the subject gets when getting grilled, a mouse is placed on a desk
in front of them and they game. No, I'm just kidding. A galvanograph is attached to their fingertips.
This uses electrical current to measure the conductivity of the skin, which increases
when damp. Next, there's the cardiospygmograph, which uses an inflatable blood pressure cuff to
amplify the sound of the subject's blood flow, and therefore heartbeat, recording blood
pressure and heart rate. Finally, there's the data acquisition system, or DAS, which collects data from each of these
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.
Modern DAS can even assist administrators through simultaneous computer analysis of
the results. Now, that's a lot of delicious, quantifiable data that's being collected, but does it
really mean that we can measure lies and solve crime?
One of the most difficult aspects of applied science is just how many potential confounding
variables there are in the real world, with the messiest being the human element.
Polygraphs measure breathing, sweating, heart rate, and blood pressure.
All physiological symptoms of stress, but there are tons of causes of stress that have
nothing to do with lying to the authorities because you just murdered your spouse.
For instance, having the authorities accuse you of murdering your spouse might be incredibly
stressful. For that matter, so is jogging, drinking coffee, watching a scary movie, performing on stage,
falling down the stairs, getting married, or being punched.
And the human body doesn't really differentiate much between these various sources of stress.
Any of them can turn you into a sweaty, stuttering mess if they're intense enough.
For blood pressure readings, there is a known problem called white coat hypertension, where
some patients have consistently higher blood pressure readings whenever they're tested
in a medical setting like a hospital or doctor's office compared to when they're tested at
home or with those automatic tests at the grocery store.
This is typically because the patient finds going to the doctor stressful.
Many innocent people then, even those who have never had a bad interaction with the
police, find interacting with law enforcement scary, and those people are more likely to
fail a polygraph. Another problem is that polygraph interpretation is, well, interpretation.
No two people respond exactly the same to all stressors.
And there's no hard line between pass and fail. Rather, administrators typically ask a series of irrelevant but uncomfortable questions to
establish a baseline, and then they compare that to later, more loaded questions about
the actual subject of the investigation. But this leaves a major opening for subversion.
Others who know how the test works may intentionally speed up their breathing during the baseline
questions or may slow their breathing during relevant questions.
Some may take anti-anxiety medication or drink alcohol before the test to mask their stress,
while others may simply walk in confident and retain their cool.
Aldrich Ames, a chronic alcoholic and a spy for the Soviet Union, was given two polygraphs
during his time at the CIA and passed both of them.
The key advice given to him by his KGB handlers? Just relax, comrade.
A major assumption of the test is that these reactions are involuntary, and to an extent
they are. But almost everyone outside of the iron lung can control their breathing at least.
And then from there, it takes a trivial amount of practice to intentionally speed up or slow
down your heart rate, which also affects your blood pressure.
In some ways, the innocent are the least prepared to take a polygraph test as habitual
liars are less likely to show signs of stress when lying.
What this all means, then, is that polygraph tests aren't terribly reliable, better than
chance perhaps, but seriously flawed. There's no firm scientific basis for using these tools to determine guilt or innocence,
and courts in most jurisdictions have ruled them inadmissible as evidence.
That hasn't stopped US law enforcement from using polygraphs as part of their hiring process,
though, nor has it stopped some researchers from trying to find a new 21st century equivalent
to the polygraph, either through MRI scans of the brain, micro-expression tracking electrodes,
eye tracking cameras, or even facial recognition technologies that's adapted to spot a lie
at a distance via security cameras. It remains to be seen, though, whether these new lie detectors will be any more accurate
than the old ones. If you enjoyed this video, do a polygraph.
Prove it. No seriously, hit the like button, and if you didn't, you can dislike.
And if you have any feedback for us, leave a comment, maybe one with a favorite outlandish
lie. Here's one from us. My heart beats twice a minute like a blue whale, and I haven't blinked once this entire
episode. If you don't believe me, go back and check.