Forensic Psychology For Dummies (43 page)

Using physiological approaches

Monitoring the physiological reactions of a suspect to detect whether he’s telling the truth or lying has been in use for some years (something I talk about in Chapter 1). The aim of the physiological approach is to pick up on what’s technically known as
arousal
: a heightened energising of the nervous system and muscles. Arousal is shown by an increase in heart rate and the rate of breathing, as well as by a change in the skin’s ability to conduct electricity because of an increase in sweating. The person’s voice can also become higher pitched and more erratic.

The problem for the forensic psychologist is that the heightened responses can also be a sign of a general emotional reactions produced by the anxiety of being questioned, especially if the person fears he’s wrongly being thought of as a liar.

Recently, a TV show demonstrated the unreliability of the physiological approach for testing lying. Celebrities were asked tricky questions to find out if the answers they were giving were true or false. The interrogator maintained that his lie-detection system showed that some of the celebrities were lying – but the celebrities hotly denied the accusation.

Likely the celebrities were reacting emotionally to the crassness of the questions being asked – in front of millions on live TV. The experiment was dropped from further shows.

Companies selling physiological lie-detector tests often claim that the lie-detector can tell the difference between testing normal anxiety and the anxiety associated with lying. For example, the claim is that as the test continues normal anxiety disappears and the emotions associated with lying show at particular points when lies are being told. But variations between individuals in their response to the questioning can mask such subtleties.

Displaying heightened emotional responses isn’t the same as lying. All physiological lie-detection procedures are in fact detecting only emotional responses, such as the anxiety of the person being interviewed thinking that he isn’t going to be believed. Because a physiological lie-detector can be unreliable in detecting whether a person is lying, few courts allow their findings as evidence. (For more on the physiological approach see the section ‘The polygraph’.)

The polygraph

The polygraph is a machine used for measuring small physiological changes in the body at one and the same time: heart rate, breathing, sweating and so on. It’s the best known procedure for detecting lying (and is usually just called a
lie-detector
). Originally, the polygraph recorded these physiological changes using pens running across a moving sheet of paper, which is where the name
polygraph
comes from, meaning many lines. Polygraph machines have since been computerised: so the magic of pens bouncing across paper with a dramatic, crunchy upsurge when a ‘lie’ was being told is no more.

In an attempt to separate normal heightened emotional responses from the responses associated with lying, the polygraph is used alongside the
guilty knowledge procedure.
Under the guilty knowledge procedure the suspect is asked simple ‘yes’ or ‘no’ questions that contain a mixture of information that only the suspect can possibly know, together with unbiased questions that act as a sort of basis for a truthful answer. The comparison between the physiological responses to the questions containing the guilty knowledge and the unbiased questions is used to detect if the person is lying.

Scientific studies show that the guilty knowledge procedure is the most reliable procedure on the market for detecting lying, although not always giving valid results. Importantly, the guilty knowledge procedure is much better at showing when a person
isn’t
lying than showing when he is.

As long as the guilty knowledge procedure spots no suspicious reactions, that person is likely to be telling the truth. So, although most courts don’t allow a polygraph test to be presented as evidence of guilt, it can sometimes be a useful way of eliminating a suspect from the investigation because of the person being shown to be telling the truth.

When setting up a polygraph test using the guilty knowledge procedure you need to know a lot about the circumstances of the crime as well as understanding how much only the suspect can possibly know (difficult if a lot of information about the crime has become public knowledge). Also, the suspect has to be carefully briefed about what happens during a polygraph test and how the procedure works.

There may not be much that only the suspect would know so other types of polygraph tests are sometimes used. For example, a suspect’s physiological reactions when presented with incriminating information may be compared with his reactions when asked innocent questions about things that many people do wrong. This is far less reliable than the guilty knowledge test. What is your likely reaction to being wired up to a polygraph and being asked bluntly ‘Have you ever lied to avoid being found out about something you did wrong?’ Like me, I think your heart is likely to start thumping and you’re gasping for breath, even though most people would be expected to say ‘yes’ to this question. And yet, a hardened criminal is quite capable of responding in a relaxed manner: ‘Yeah. Sure. Haven’t you?’

There are many ways of cheating on the polygraph test (websites are devoted to telling you how to do so). In general, if a person’s emotional responses are haphazard, or they set up some distraction such as having a stone in their shoe that’s hurting them, or because they’re having difficulty in focusing on the question, the polygraph machine is unable to detect the difference in responses to crucial questions.

Anyone making a living out of polygraph testing tells you that the key to a valid test is in establishing a working relationship with the person being tested. Having rapport with your subject determines how much he believes in the result of the procedure, sometimes to the point of admitting to his guilt without even needing to read the output.

Voice stress analysis

Voice stress analysis
is a recent computer product on the market for detecting if a person is telling you the truth while speaking on the phone. The product is controversial and mostly used by companies dealing with insurance claims for detecting if the customer is making a false claim. The idea behind voice stress analysis is that any sound has measurable
frequency
. A sound with a high pitch has a rapid frequency; a low pitch a much slower frequency. So, an analysis of the frequency picks up any heightened emotional responses in the caller’s voice.