Hey there.
Welcome to "Brain Stuff."
I'm Jonathan Strickland.
Today's question is, how
do polygraph machines work?
Well, those are lie
detector machines.
We need to define some things.
"Poly" means "many."
It's from the Greek
word for many.
And in this case, the "many"
refers to the many sensors
used by the machine.
Now these sensors detect
changes in your body-- things
like your heart rate,
or your blood pressure,
or whether or not you're
sweating, or maybe
if you're taking
deeper, shallow breaths.
Now the "graph"
part of polygraph
comes from the Greek
word meaning "writing."
And in the case of this
machine, the writing
is talking about that
strip of paper-- you know,
where you see the
needle making the line
or maybe there's
several needles.
That would represent
the actual changes
being detected by the sensors
that are attached to your body.
So as your body reacts
in different ways,
it's recorded on
this piece of paper.
If you have a little
reaction, it's a smooth line.
Big reactions are big jumps.
Now more modern
polygraph machines
actually use digital
readouts rather than
that piece of paper-- though
some people prefer the paper
because it has a sort
of psychological effect.
So how does this actually work?
You get an interrogator asking
someone various questions.
First they ask a few
baseline questions
to get the normal response
to being interrogated.
Is your name
Jonathan Strickland?
Yes.
Then the interrogator
alternates between what
are called target questions
and other baseline questions.
Do you have facial hair?
Yes.
Have you showered today?
Yes.
Huh.
Now, after the questioning,
the interrogator
starts looking at those
target questions-- looking
for big signs of activity.
That indicates that the
person being interviewed
has had a stress
response to being asked
that question, which
could indicate a lie.
But here's a problem.
This approach assumes
a standard response
to lying, or even
being interrogated,
which isn't how
humans work actually.
People may have elevated signs
simply through experiencing
stress while being questioned.
Other people-- you know,
the cool customers,
the sociopaths-- they might
be able to just spout off
outrageous lies without so
much as a blip on the radar.
Now ultimately, lie detector
tests are a poor tool
to judge if someone's lying.
They really just judge
if they're uncomfortable.
What they're really, really
good at is coaxing confessions.
You see, if a person thinks
that the polygraph machine is
actually working, they may be
worried about being caught out
in a lie, and be more
inclined to come clean.
But as for a
scientifically sound way
of determining whether or not
someone is actually lying,
it's a bust.
I Now this has led some,
but not all, countries
to rule that
polygraph evidence is
inadmissible in a court of law.
Ours is not necessarily
one of those.
So that's how polygraphs work.
Or how they don't
work, depending
upon your point of view.
Well, I hope you
enjoyed this video.
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We're going to be answering
lots more questions pretty soon.
That's the truth.
