Question

What do "lie detectors" actually detect?

Answer 1

They detect physical signs of emotional arousal.
Like....increased sweating, and increases in heart rate.

When people lie, they tend to sweat a little more.
The pupils in their eyes dilate (widen).
And their heart rate increases.

This is all because of stress and anxiety.

The bigger the lie, the more anxiety a liar would feel.
ESPECIALLY if the liar has a lot to lose, if they cannot successfully lie.

FUN FACT: This is why psychopaths are excellent liars. Psychopaths don't feel stress and anxiety when they lie.