Attempts to fool the public into mistakenly believing that lie detectors work do not make for either good law enforcement or sound public policy.
If you are ever unfortunate enough to take a polygraph lie-detector test, you will be told by its administrator to answer a question falsely. For instance, you might be told to say ‘no’ when asked if it is Thursday, even though it is. This is to establish a baseline for lying, the examiner will say. Even though you may feel very little guilt about doing just as you are told, the sensors strapped to your body will register changes in pulse, breathing and sweating. Or so you will be informed.
In fact, the polygrapher does not even need to read the traces, because they are unlikely to show anything, according to polygraph training manuals. The purpose of this ‘test’ is to establish in your mind the infallibility of the machine — if you lie, it will catch you.
As a result, a polygraph exam is only as useful as the examinee thinks it is. A nervous subject is more likely to betray a lie as much through intonation or behavioural changes as through any physiological parameters. Polygraphers are trained to pick up on such cues, making the polygraph a useful tool for interrogation.
But that's not the same as calling it a lie detector. In fact, the instrument's reliability in detecting lies has been thoroughly discredited, most recently after an exhaustive review by the US National Academy of Sciences. In a 2003 report, an academy panel concluded that the tool was worse than useless for catching spies — its main purported national-security function. Spies can learn to defeat the machine, the panel pointed out, and their passing score merely serves to bolster their credibility.
The Department of Energy responded by reducing the number of employees at US nuclear weapons laboratories who are required to take the exam. But other US government agencies have been slow to follow suit, arguing that the polygraph remains useful to them.
New detection technologies are being rolled out in response to current security concerns in the United States (see page 692). These include devices that monitor brainwaves and cameras that read heat signatures in the face. But there are alarming signs that the inventors of these tools are more interested in getting them into the field than in doing the research needed to see if they work.
As with the polygraph, the problem lies with the seductive appeal of technology. In the courtroom, juries are far less likely to question the results of a ‘scientific’ test than the testimony of a witness. Consequently, the polygraph is now banned as evidence in most states.
If the purpose of these tests is really intimidation, then any technology will do, as long as subjects believe that it works. But if the goal is to discover deceit, then the technology must be validated by solid research. If this is to be done, the US government will have to engage independent scientists — rather than people who work for the companies marketing the machines, or the agencies that plan to deploy them, as is currently the case. Only then will the truth come out.
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True lies. Nature 428, 679 (2004). https://doi.org/10.1038/428679a