What are the current implications of a 100% accurate lie detector? Would we use it in the justice system? Would it change the way we chose and elected leaders?

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    $\begingroup$ Even if it was 100% accurate, you won't be able to prove that it is and thus people can blame it by lying. $\endgroup$ Aug 11 '17 at 16:22
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    $\begingroup$ A perfectly accurate lie detector cannot tell whether a person tells the truth or not, only whether the person tells what they believe it's the truth or not. This is a plot point in many fantastic adventure novels. Not to mention that a skilled operator can try to answer truthfully but misleadingly -- this is a plot point in the Safehold series by David Weber and in the Belisarius series (by David Drake and Eric Flint). $\endgroup$
    – AlexP
    Aug 11 '17 at 16:30
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    $\begingroup$ One huge implication would be the philosophical and religious effects of knowing that the definition of a "lie" is so definitively clear that a 100% accurate lie detector can exist. Politicians would also have to get really clever, pushing the bounds of whatever this definition is. $\endgroup$
    – Cort Ammon
    Aug 11 '17 at 16:40
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    $\begingroup$ If you could focus it on one aspect; the justice system or elections, maybe it could be salvaged, but as written it is way too broad a question. $\endgroup$
    – Josh King
    Aug 11 '17 at 16:49
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    $\begingroup$ Welcome to WorldBuilding Chess! What you are asking is incredibly broad. It looks like you would like to have a long list of all the ways this could affect societyas you list two very distinct topics already, that are pretty broad on their own. As a rule of thumb "If you can write a whole book to answer the question it is likely too broad". Please edit your question to narrow the scope down. Otherwise it might get closed until you edit it to conform to the sites guidelines. If you have a moment please take the tour and visit the help center to learn more about the site. Have fun! $\endgroup$
    – Secespitus
    Aug 11 '17 at 19:27

Science isn't always applied equally

We know genetic tests are far more accurate and reliable than eyewitness testimony. Yet we still use eyewitness testimony -- and often value it more than genetic data -- in jury trials.

Your perfect lie detector may not be "trusted" by jurors, despite every logical reason to believe it is perfect.

There's truth and then there's truth

There's a concept in Robert Jordan's Wheel of Time series. The female magicians (called Aes Sedai) of that book series are magically oath-bound to never lie. Which gives rise to the idea of "An Aes Sedai never lies, but the truth she speaks may not be the truth you think you hear."1

If you cannot tell a lie, that's nowhere near the same thing as not deceiving your audience. One can manipulate the truth such that it doesn't mean what people thinks it means. And there will always be people who figure out how to do just that.

People will swear something is true, even when it isn't. A good investigator must be very careful how they question witnesses to make sure they don't taint the witness or lead them to misremember data. They don't ask "did you see a redheaded person robbing the store?" but instead "What color hair did the robber have?" Asking a Christian to say "Jesus Christ is God" will give you a Truth. But asking an atheist the same question will give you a False. So even this truth/not truth data must be managed carefully.

Partial truths, shades of the truth, what someone believes to be true, or just outright misdirection, can all pass a "100% truth" test in some cases. And good lawyers would both know how to use this and how to taint a jury by making them question whether the other lawyer was using this.

Fear of false positives?

You may need to expand on how your detector works. But the process for modern-day polygraphs is a very careful process. This is because false positives are not impossible.

I've taken a polygraph exam. The process isn't what you see in TV shows or movies. The examiner sits you down, and vary carefully goes through all the questions before you even see the machine. The examiner explains the process, and then fine-tunes the questions to help rule out false positives. One example from my exam was this baseline question: "Have you ever stolen from a previous employer?" So she asked me that. And we discussed what the question meant, discussed the reality of office environments, in a back-and-forth ("Does taking paperclips count? What about printing personal things on office printers?") that refined the baseline question to the one in the final exam: "Have you ever stolen anything worth more than $100 from a previous employer?"

Then, once all the questions have gone through this back-and-forth, you sit for the exam. In an unlit room. Where no external distractions can exist -- no hallway noises, etc. -- again, because false positives. And the examiner goes through the questions three times. Three times because that builds an average. Again, to reduce those false positives.

So again, unless we're talking about magic spells, no such instrument would be useful under the high stress environment that a court room or presidential debate would be.

Final outcome

It would make job screenings more useful. And might help criminal investigations. But probably wouldn't impact courts or elected officials much. Again, unless we're talking about magic. Because then all bets are off.


Letting aside 100% is not something actually existing in Nature, all problem is in the definition of "lie".

Please consider the famous citation (Robert A. Heinlein):

“It’s not enough to be able to lie with a straight face; anybody with enough gall to raise on a busted flush can do that. The first way to lie artistically is to tell the truth — but not all of it. The second way involves telling the truth, too, but is harder: Tell the exact truth and maybe all of it…but tell it so unconvincingly that your listener is sure you are lying.”

How would your "perfect lie detector" handle this? Only way out would be to actually detect the "intention" to lie, but that is a bit over the board.

Consider also that best liars actually convince themselves the lie is actually true (this is how certain individuals "resist" even drugs).

This said a few considerations:

  • Experienced liars would still escape, is some way, detection.
  • Widespread usage would face a heavy opposition because, in spite of our innate (and sometimes morbid) curiosity everybody has something to hide, in many cases with good reason (who would save Santa Claus?).
  • All religions would fiercely oppose it (even if majority(?) of "ministers" really believe there are many that do not or, at least, hold some doubts).
  • Lawyers would fiercely oppose it because they would become useless.
  • Politicians wouldn't see it well (unless they happen to be in one of the above "slippery" categories).

Given human nature this wonderful invention is likely to be buried, together with its creator, in some very deep pit (after public burning at the stake).


I presume that this lie detector can scan almost all the human brain, because lying detectors can be decieved if the person repeat the lie to itself a lot and be a very good actor. Maybe you've repeated the same story so much with your own lies that you truly believe them. The person who asks the questions has to be skillful too. The questions and answers would be analyzed in the end of the sessions and they will try to find "the Truth" based on their own hypothesis of what is not true. (Because, again, this is a LIE detector. There could not be a truth detector simply because what is true for you can be false to me.)

  • Corruption would only happen by human mistakes and misunderstandings.
  • You cannot extract the truth, but only, what is not true.
  • This could change diplomacies as a whole and even wars could escalate as secret plans can be revealed.
  • Not every country would acept this kind of machine and will try to scam it at all costs. They could use scientists to question the integrity of this detector. (They could get some hipsters or nerds from here e_e)

  • You must also think on the price, because if it is too low and
    democratic, the whole society with its families and individuals could be affected. Hippie-like movements would emerge praying for a cleaner humanity.

  • Abstract questions can be hard to measure as it involves doubt, not believing and faith.

  • The lie detector could still be used for evil if the analyzers creates crazy theories of the questioned individual.

  • There would be burocracy and laws for the lie detector, because the analyzer could ask a hundred of questions and the interpretation of what is true based on what is not true can change from analyzer to analyzer.
  • Sciences and philosophies could born because of this.

"Do black swans exist?"


The lying detector did not ring.

"Where have you seem them?"

"In various movies. So, I suppose they exist... isnt it?"

The lying detector did not ring.

"No, they dont."

Another guy name's Popper enters the room and says "Just because you havent seem black swans does not mean they do not exist."

So what is True?


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