Many people know that lie detector tests are bullshit. But the video above, from Adam Conover at TruTV, does a pretty good job of running through the reasons why the tests are so bad.

The video also demonstrates another fact some Americans might not be aware of: In many cases, police actually know that the test doesn't really detect if a suspect is lying — but use it anyway to trick suspects into (sometimes false) confessions.

"If the examiner does the theater well, and tricks the subject into believing that his or her lies can be detected, they might confess," Leonard Saxe, a psychologist at Brandeis University who's conducted research into polygraphs, previously told Vox.

So how accurate are polygraphs? The American Polygraph Association claims the tests are around 90 percent accurate, but this group doesn't exactly have an interest in being honest about the tests if they are terribly inaccurate, since it advocates for the use of the polygraph.

"There is no evidence that any pattern of physiological reactions is unique to deception"

Other experts and research are much more skeptical of the polygraph's validity, since the test really measures physiological indicators of anxiety, not honesty. As the American Psychological Association notes, "There is no evidence that any pattern of physiological reactions is unique to deception. An honest person may be nervous when answering truthfully and a dishonest person may be non-anxious. Also, there are few good studies that validate the ability of polygraph procedures to detect deception."

In the criminal justice system, the goal is to see if a suspect is guilty beyond reasonable doubt. But since there's a very good possibility that someone was simply nervous when the test deemed him a liar or abnormally calm when the test deemed him honest, and there's no scientific evidence that the tests are valid, there's a very strong case for reasonably doubting any polygraph results. Polygraphs are, in other words, not very useful at actually doing what they seek to do.

The good news is you're never required to actually take a polygraph during a criminal investigation. So if police ask you to take the test, or even try to trick you into thinking it's mandatory, it might be a good idea to refuse — to avoid self-incriminating yourself. (But don't take my word for it — consult a lawyer on this.)

Still, polygraphs continue being used not just in criminal justice settings, but also by some government employers. About 70,000 people a year undergo such tests while seeking security clearances and jobs with the federal government, even though a 1988 law bans private employers from putting their job applicants through the same process. Maybe it's time to just dump these machines.

Watch: The psychology of police sketches