Polygraphs measure respiration, heart rate, blood pressure, and skin conductivity. A lot of claims are made about the accuracy of polygraphs, but what they really measure are your level of anxiety. The operator asks you a series of questions. Answers that make you uncomfortable cause you to sweat slightly (therefore changing your skin conductivity), cause your blood pressure to rise, your heart rate to change, and your breathing to change.

The rest is subjective.

A polygraph usually begins with an interview. You're asked a series of questions, things that are specific to the subject you're being grilled on, and general things about your credibility. ("As a child, did you ever attempt to mislead authority figures?") The operator also attempts to establish his or her own credibility and training, and that of polygraphs in general. ("When I worked for the US Government, I was responsbile for catching *3* KGB agents")

As you sit in a chair, sensors attached to your body are hooked up to a computer, or the old fashioned analog type of machine like you see on tv. ("This computer uses *three* *different* algorithms to decide whether you're lying") There's a band around your chest to measure breathing, a clip on your finger to measure skin conductivity, and a blood pressure cuff around your arm which measures pulse and blood pressure.

You're asked to close your eyes, remain still, and breathe consistently while you're asked questions. If you, say, pause in your breathing, the operator may accuse you of trying to "deceive the machine".

Often, the operator will try to establish a baseline for your response by telling you to deliberately lie. ("I'm going to ask you your name. Respond by saying, 'My name is x'"; where x != your name) Then they'll go through the questions (including things like "Do you intend to lie to me today?") two or three times.

The rest is up to the operator.

Most of the stuff out there about beating polygraphs is a crock of shit. (Then again, so are polygraphs.) The polygraph operators have read all the books, pamphlets, and web pages with all the techniques. They even ask you if you've read about it.

re: How to Beat a Lie Detector -- on one occasion, the operator made me remove my shoes and felt my feet to see if I was using this technique.

Also, there are a number of cases where polygraphs *are* allowed as evidence in US courts. Rules vary from state to state.

Source: personal experience (failed one, passed another)

For my final thesis in electronic engineering I created a portable stress recorder that checked a patient's galvanic skin response, breath rate and heart rate variability.

After a while I realized that my gadget could double as a polygraph (lie detectors rely mostly on the fact that telling a lie will cause some stress, such as fear of being caught).

I'm rather sure that one of the algorithms used in professional polygraphs involves the heart rate variability. You measure the time between two QRS complexes (that's the time between two heartbeats), apply some windowing to the data and then feed the result to a Fourier transform. High-frequency peaks in the resulting graph are related to parasympathetic activity, while low-frequency peaks are linked to the sympathetic system. (VERY LOOSELY speaking, this detects small irregularities in the rhythm. The more stressed you are, the more irregularly your heart beats, creating a high-frequency peak).

I was rather surprised to see that the circuit could tell if I was standing, sitting or lying down - the high frequency peak showed up quite clearly in the first case, and was nonexistant in the third. Another way to test the system was to hook somebody up and make him count down from 1000 in steps of 13, as quickly as possible (a psychology text suggested that, halfway through the count, the examiner should check his watch and comment that all the other test subjects were faster - just to make the poor victim more nervous!)

I'm not saying that lie detectors work; quite the opposite, it's so easy to induce a stress response in your body that the examiner will have a hard time distinguishing them from your lies. I wholly agree with narzo's writeup.


Amusing anecdote: my gadget had a 4-digits hexadecimal display to show its internal status; each digit checked a different subsystem. In my very first test I glued the electrodes to my chest and wrist, pushed the ON button and received the shock of my life when the display cheerfully informed me that I was "DEAD"...

Next time I'll stick to the decimal system.

Pol"y*graph (?), n. [Gr. writing much; much, many + to write: cf. F. polygraphe.]

1.

An instrument for multiplying copies of a writing; a manifold writer; a copying machine.

2.

In bibliography, a collection of different works, either by one or several authors.

Brande & C.

 

© Webster 1913.

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