The Truth Behind Lie Detectors

Lying. You do it. I do it. We all do it. Whether it’s just a little white lie or an outright, in your face deception no one has ever gone a lifetime without telling porkies. Most of the time it’s harmless, like perpetuating the Santa Claus myth or saying that your significant other’s new haircut looks really good. Obviously the more significant times we lie are when we’ve been caught out doing something bad and we’re trying to get out of trouble, like when we’re caught saying that we lied about our significant other’s new haircut looking really good, or when we kill someone, for example.

Is it possible, though, to tell when someone is deliberately distorting the truth? 2500 members of the American Polygraph Association say that it is; and with 90% accuracy. That’s pretty awesome. Or at least it would be if it were true. The reality, you might say: the truth, is that polygraphs, or lie detector tests, are very unreliable and certainly aren’t capable of establishing veracity.

First of all, let’s be clear about what a polygraph test is. The device hasn’t really changed in nearly a century. It is a box with various sensors inside it able to measure three basic physiological factors. Straps around your chest measure your respiration rate; a cuff around the arm measures blood pressure and/or heart rate and electrodes on the fingertips measure skin conductivity i.e. how sweaty you are. One of the very first devices was invented by a man called Leonarde Keeler in the 1930s. Even then, however, it wasn’t anything especially novel; these measures had been in use for decades already going back into the 19th century, all that had happened was that all three pieces of equipment had been lumped into one box.

The problem with these very basic tests is that they aren’t specific to telling lies. Most of the time we let our subconscious deal with the tedium of constantly breathing but we’re all able to take conscious control whenever it suits us. Some people can control their heart rate to a reasonable degree, making it go up is certainly no great difficulty for most people. In films we see crooks putting a drawing pin in their shoe or clenching their sphincter to throw the device off and, apparently, these and a bunch of other methods can all successfully scupper an interrogation.

One of the main problems with the technique is that of false positives, where a person telling the truth is falsely accused of lying. This is because the three indicators used might not be much use for detecting falsehoods but they’re pretty handy at detecting anxiety and fear. It isn’t hard to imagine a scenario where an innocent person is interrogated about a crime and they’re so anxious about it that they become especially stressed when asked questions pertinent to the offence. These cues could easily be interpreted as an attempt to conceal facts.

It is no wonder, then, that polygraph tests have not been admissible in federal courts and most state courts since the 1920s; even then it was clear that the science behind lie detectors was shaky at best. Since then there has been plenty of in depth analyses of the available data to test the effectiveness of polygraphs.

In 2004 the American Psychological Association reviewed the available data. “…the idea that we can detect a person’s veracity by monitoring psychophysiological changes is more myth than reality,” they revealed, concluding, “For now, although the idea of a lie detector may be comforting, the most practical advice is to remain skeptical about any conclusion wrung from a polygraph.”

In 2003 the National Academies of Science published a 400 page report on lie detectors having evaluated more than a century of evidence. “The theoretical rationale for the polygraph is quite weak.” “The inherent ambiguity of the physiological measures used in the polygraph suggest that further investments in improving polygraph technique and interpretation will bring only modest improvements in accuracy.” You get the picture.

It is not a little worrying, then, that polygraph testing remains prevalent throughout the United States. Though it won’t be allowed in court this does not stop police forces using it routinely. As a piece of dramatic theatre and showmanship it is second to none in eliciting false confessions, and confessions that are obtained during a polygraph are admissible in court.

There are well publicised instances of innocent people falsely confessing during a polygraph and being convicted, and also of stone cold killers successfully passing a polygraph by doing nothing more than ‘relaxing‘.

It is certainly positive, then, that they remain broadly inadmissible in courts, but it would be even better if they were removed from law enforcement practice all together. It seems unlikely that this will happen anytime soon, though. There’s no appetite for it. There is a perception amongst the general public that they work, no doubt encouraged by their use on trashy daytime television shows where their results are taken as gospel. The status quo seems unlikely to shift in the foreseeable future.


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