What really happens to your body when you take a polygraph test

Although they lack the pizzazz of lightsabers, polygraphs — better known as lie detectors — work on the equally fantastical premise that a machine can definitively say whether the subject of the test is lying or not. The Supreme Court ruled against them in 1998, dismissing them as inadmissible evidence in court martial hearings, though they still left their use in court hearings to the discretion of the state. In fact, there are plenty of ways to beat a lie detector test, so, despite Wired's report that 2.5 million polygraphs are given a year as part of a $2 billion industry, they should be treated with some degree of justified skepticism.  

However, to be balanced, it should also be pointed out that there is a possible use for polygraphs. In a 2007 study conducted by the University of Kent, researchers found that the majority of sex offenders who were surprised with a polygraph test tended to be more open about admitting to their offenses than those interviewed without a test. So, while a lie detector does not detect lies, it can persuade people to be truthful anyway. On the other hand, to balance that balance, if this were to become standard practice, people would start lying again, anyway, so the whole potential benefit goes out the window. 

But what polys are graphed?

If a lie detector can't tell if someone is lying, what does it do?

Well, the word polygraph gives the best explanation. Poly means "many," and graph means "writing," and the original machine was called this because it allowed someone to write a document and its copy simultaneously. Back in the 18th century, this technology was almost as fantastical as lightsabers.

By the 19th century, as Steven Poole notes in his word column in the Guardian, it came to mean measuring multiple types of body measurements simultaneously. So, someone could take your pulse and your respiration at the same time. Being able to take these measurements at the same time transformed into the polygraph or the lie detector test you know today, which measures involuntary signs of stress, sweat, heightened blood pressure, sharper breathing, and an increased pulse. The idea was that lying is stressful, so the machine would see whether a person becomes stressed when answering. However, being interrogated is stressful enough on its own, and people expecting to be interrogated can practice beforehand, so you end up with a system where some innocent person gets branded a liar because they respond naturally, while a potential serial killer walks free for having a level of self-control that many would consider somewhat psychopathic. Perfect.