What Is Polygraph Testing?

What Is Polygraph Testing

A polygraph machine is used to attempt to detect physiological changes that are thought to occur when a person tells a lie.

These measures include the blood pressure, the amount of sweating on the palms and the heart rate. Polygraph testing is currently being used to investigate crime in a number of countries around the world, including the USA, Japan, South Korea and Israel (Raskin, 1990). At present there is a wide-ranging literature evaluating the use of the polygraph machine and associated techniques, largely based on laboratory experimentation.

Like any psychometric test, the use of a polygraph machine in the detection of deception has been questioned on the basis of its reliability and its validity. Reliability refers to the ability of a test to be consistent, no matter who is carrying it out. The question of validity asks whether the test is actually measuring what it claims to measure. The answer to both of these questions is vital in answering whether the polygraph should be introduced into British policing.

The polygraph machine has been used in a number of different ways to elicit useful information from suspects. Bull, Gudjonsson, Hampson, Baron, Rippon, & Vrij (2004) identify four main techniques: the Relevant/Irrelevant Technique, the Directed Lie Test, the Control Question Test, and the Guilty Knowledge Test. Of these, the majority of research has addressed the last two and so the discussion will concentrate on these.

The Control Question Test

The theory behind the Control Question Test (CQT) is that the physiological responses of a suspect to control questions are compared with those which are directly relevant to the crime. Control questions are specifically chosen to be vague in nature and to relate only indirectly to the crime under investigation (Iacono & Patrick, 1997). This means that they should provoke high levels of physiological arousal in innocent suspects as they are designed to elicit guilty memories - but those that are not under investigation.

By contrast, to an innocent interviewee, the specific questions about the crime should evoke lower physiological arousal as they can be categorically denied. To the guilty interviewee, however, the reverse pattern should be seen with higher physiological response seen to the more specific questions.

The basic paradigm for assessing the polygraph test used in laboratory investigations involves a 'mock crime' with participants randomly told to act either innocent or guilty. Raskin (1982), for example, explains that the guilty participants enact the mock crime, while the innocent participants simply have the facts relayed to them. Both groups are given a cash incentive to pass the test, and this goes some way towards giving them the required motivation to pass the test. Many of the earlier studies used the CQT test and found some encouraging results. Carroll (1988) summarises some of these studies, referring first to the Office of Technology Assessment of the