... don't work.
There's been suggestions that terrorists leaving prison, should be put through a lie detector test to see if they intent to commit further offences.
The problem is, lie detectors don't work.
They are great fun when used on reality TV shows, and can lead to great drama and confrontations. But they don't actually work.
The American Psychological Association says they don't work. "Most psychologists agree that there is little evidence that polygraph tests can accurately detect lies."
Yes, it would be great if there existed a "magic lasso" as used by Wonder Woman, that forced people to only tell the truth. Or if there was a "truth serum" (why is it always called a serum?) that did that. Or if there were some technology that could tell if someone is lying.
William Moulton Marston (author of the Wonder Woman stories) invented the lie detector.
Since then, it's been used many times, both for fun (as in reality TV shows) and in serious situations, and people seem to think that it works.
It doesn't work.