A running theme through the show Black Mirror are these small devices that people put on their temples. In one particular episode, an insurance rep is using the device, along with a little TV that shows what the person wearing the device is thinking, to get the details about someone getting hit by a delivery truck. I won't ruin the episode for you, but essentially someone tries to lie and they get caught because the insurance rep could see what they were thinking. How cool of a device would that be in real life?!
The closest thing that humans have been able to come up with to detect deception thus far is the polygraph, which, although it's a cool device, doesn't do what it purports to do. The polygraph claims to be a lie detector test by measuring things like skin conductivity (sweat on your skin), heart rate, blood pressure, and breathing, and then the examiner interprets the results and decides if you were lying or not. The idea is that there are different things that the human body will do when we lie, like sweating and our heart rates going up. This would be a great invention, if any of that were true. Unfortunately, no evidence has been found to suggest that there are any consistent features that people exhibit, psychologically or physically, when they're lying.
When police put too much weight into whether someone passes a polygraph test, it seems that (based on the 1000s of hours I've put into true crime podcasts over the last several years) they can disregard other evidence. I'm definitely not saying this happens all the time, but I'm never surprised when I hear about cases getting fumbled because of the reliance on a polygraph test. To be fair to the polygraph, these cases do tend to involve other shoddy police work and investigative tactics.
One particular case where trusting the polygraph let a horrible murderer stay out of prison for more than 15 years, is the case of Gary Ridgway, or the Green River Killer. He passed a polygraph test in 1984 and was dismissed as a suspect in the case of the murders of several sex workers. At the time, the police were focusing on another suspect who was, of course, innocent, but who failed a polygraph test. It took until 2001 for him to finally be arrested and imprisoned, using DNA samples he provided in 1987. Although most of his murders occurred before he took the polygraph test, he was able to kill more women between then and his arrest in 2001.
On the other side of this, imagine a close loved-one going missing and you being asked to take a polygraph test. You would probably be in a pretty intense state of worry and despair, so what are the odds you're going to be able to sit still, have your heart beat regularly, and breathe normally while being asked questions about your missing loved-one? Probably not super high, right? Taking a polygraph test can make people anxious! And what happens when we get anxious? We sweat, our heart rates and blood pressure goes up, and our breathing becomes irregular. I mean, I have regular ol' anxiety and I doubt there's a situation where I take a polygraph test and pass.
Luckily for us (aside from the cold winters, horrible housing market, and extremely poor treatment of Indigenous Peoples), we're in Canada, and polygraph tests aren't admissible as evidence in court, however, it's still able to be used in the investigative process. Our southern sis' on the other hand, still allows polygraph tests as evidence in certain states, and their National Security Agency even made a video promoting the polygraph. In true American fashion (said as an embarrassed American citizen), instead of looking at the readily available scientific evidence showing polygraph tests are garbage, they indicted federal employees who had any ties to someone claiming to be able to teach you how to pass a polygraph test.
So, if you ever find yourself in a situation where you need to take a polygraph test, make sure you know how to pass (and probably have a lawyer on stand-by).