What would happen if a handheld gizmo was invented that could
analyze voice stress or something and tell if a person making a statement was consciously lying, with perfect accuracy? How would society change?
Trials would be simpler. I imagine that a great deal of the social lying that I personally despise would become unfashionable.
She: Does this dress make me look fat?
He: Um… no?
-BZZT-
Would this device make things better? Worse? Would the govt. try to outlaw it?
If such a device existed, people would instead have to learn a mental discipline similar to Orwell’s “Doublethink”, in which they would temporarily make themselves consciously believe every lie before they said it. If this lie-detector device were ubiquitous, they’d probably pick up this skill while they were still children (“Hey, wow, I just fooled a lie detector! Lemme see if I can do it again!”).
Of course, we could just have Tony Robbins hypnotize us to see everybody’s Inner Beauty instead. 
The first thing is that every Government would buy up the technology, to avoid the following.
Voter (armed with truthteller) “Do you have our best interests at heart?”
Politician (with hunted look) “Errr, yes?”
Buzz!
Voter “Are you, in fact, a lying scumbag who is only in it for the power and money, and who will do anything to get re-elected?”
…and you can guess what happens next. :rolleyes:
I think people would simply say less.
In the example you give, I think the woman is seeking reassurance rather more than a statement of fact.
After all, she already has a truthteller - a weighing machine.
So I would go for:
She: Does this dress make me look fat?
He: (thinking carefully that your body already looks fat) No!
or more kindly:
She: Does this dress make me look fat?
He: (thinking carefully she is the love of his life and that beauty is in the eye of the beholder) No!
Dear God!
Society would Cumble, there’d be anarchy in the streets, all human life would slowly self disintegrate, until the horrors forced us all to ban them as devices of Satan incarnate.
The politicians would have perpetual nightmares until this technology was under the exclusive control of the government.
Plausible deniability would become everything. It would be easy for a person to speak a falsehood if they didn’t really know it to be false.
This may not necessarily improve courtroom proceedings. If an eyewitness sincerely believes her account of a crime’s details are accurate, even when it is false, then no such alarm would be raised.
Well, I’d have two fewer sister-in-law for a start, and - not entirely unrelated - fewer nephews and nieces.
It is very easy to mislead people without lying.
The most effective way to lie is to not be conscious of lying.
Many, many people can detect a bald-faced lie when they see one without any device, if not consciously then sub-consciously. I would even go so far as to say that telling a bald-faced lie is foolish because you are likely tipping your hand or focusing attention in a bad area.
Not to mention carrying around such a device would be in bad taste in social circumstances and a sign of distrust in most other circumstances.
The best way to avoid lying is to ignore or deflect direct questions. Most politicians are practiced at this.
In general, a good way to mislead someone is to focus their attention on something irrelevant and then say something vague or do something that might lead them to draw their own conclusion. The best lies need no words, and are executed sub-consciously.
Atreyu, but such questioning would go by the wayside when they need only ask their suspects, “Did you do this?” Trial over. In fact, I can’t even see the need for a jury anymore, except perhaps to ensure that the machine is working!
It is very easy to mislead people without lying.
The most effective way to lie is to not be conscious of lying.
Many, many people can detect a bald-faced lie when they see one without any device, if not consciously then sub-consciously. I would even go so far as to say that telling a bald-faced lie is foolish because you are likely tipping your hand or focusing attention in a bad area.
Not to mention carrying around such a device would be in bad taste in social circumstances and a sign of distrust in most other circumstances.
The best way to avoid lying is to ignore or deflect direct questions. Most politicians are practiced at this.
In general, a good way to mislead someone is to focus their attention on something irrelevant and then say something vague or do something that might lead them to draw their own conclusion. The best lies need no words, and are executed sub-consciously.
It is not a lie if you believe it to be true.
All I know is, the first person to invent a gizmo to defeat the gizmo would be insanely rich. 
James Halperin’s book The Truth Machine addressed this question. Apparently it’s soon to be a major motion picture.
The Truth Machine does not appear in the Internet Movie Database.
Until the movie appears there as being “in production,” I wouldn’t hold my breath about it “soon” becoming a major motion picture.