6 September 2009

Lie Detection Anyone?

This is a short essay that I wrote as part of a final psychophysiology exam.
The question asked for the pros and cons of using psychophysiology for lie detection in relation to an article on psychophysiological modelling. It was only worth 5% and I got full marks for it.
This is one of my passions. I'm fascinated by Event Related Potentials (ERPs), Electroencephalography (EEG) for the measure of mirror neuron activity, Electromyography (EMG) in the measure of attitudes and in general, I have an interest in any physiological response to psychological events.
I'd love to be able to complete my Honours year in the field of social neuroscience but I'm worried about being away from the workplace for too long. Hopefully in the future...

This essay uses arguments set forward by Cacioppo, Tassinary and Berntson (2007) in relation to psychophysiological relationship to outline the pros and cons of the use of psychophysiological measures in lie detection. In psychophysiology, a polygraph is often used to measure the increase in arousal resulting from lying and eliciting changes in heart rate, blood volume, skin conductance and respiration.

Cacioppo et al. (2007) argue that simply knowing that manipulating a particular element in the psychological domain leads to a particular psychophysiological response does not enable one to infer anything about the former based on observations of the latter. If we look at this in the context of the Guilt Knowledge Test, a subject’s response to a question relating to the crime when the subject is guilty would represent the psychological domain. Meanwhile, any significant rise in the suspect’s skin conductance would represent the psychophysiological response. What Cacioppo et al. argue is that it is not possible to assume with complete certainty that other psychological reasons might not have led to the observed physiological response. In lie detection, the skin conductance changes assumed to characterise the suspect’s increased arousal when they recognise information only a guilty person would be expected to know, may be a psychophysiological outcome resulting from other psychological states. This is one disadvantage of using psychophysiological measures for lie detection. Notably, there is a many to one relationship characterising lie detection. It implies that responses measured by the polygraph could arise from any other psychological state rather than the assumed guilt state. It has been found that some people can manipulate their own breathing, press their toes on the floor or bite their tongue, thus affecting polygraph measures. For example, Honts, Hodes and Raskin (1985) tested the accuracy of the Control Question Test (CQT) and found that guilty persons in a mock-crime experiment could produce enhanced responses to control questions and therefore be classified as innocent.

Using psychophysiological measures for lie detection can be advantageous if the relationship between the suspect’s state and their psychophysiological response can be defined as one-to-one. In this form of relationship described by Cacioppo et al. (2007), only one physiological response predicts a psychological event within a given context. The psychophysiological response becomes known as a marker and its presence is so strictly associated with a particular condition that its presence is more accurately indicative of the presence of this condition. In other words, presence of this marker in lie detection increases the accuracy of the decision about whether or not the suspect is guilty. The measure of event related potentials (ERPs) for lie detection may be the closest thing to this form of relationship. One of these event related potentials, the P300, only occurs where a subject is presented with deviant or interesting information. In the lie detection paradigm, this late P300 ERP can manifest if the subject is presented with information that only a guilty person would recognise. According to Cacioppo et al.’s terms, the presence of the P300 can be construed as a strict marker for suspect recognition of crime related stimuli. In this context, the presence of a P300 in a suspect’s EEG recording can indicate that the stimulus presented was relevant to the guilty person. Conversely, absence of the P300 can indicate that the same stimulus presented was irrelevant to the therefore innocent person. A study by Farwell and Donchin (1991) which measured the P300 response in a fictional spy scenario found that there were no false positives or false negatives and that wherever a decision could be made based on the P300, it was accurate. In a second experiment, target stimuli related to a given person’s offence produced large P300s while stimuli not related to the offence produced only a very small or no P300 at all. Overall, the determinations of this second experiment were 100% correct. It can be concluded that use of ERP measures offers promising results in the accuracy of lie detection making psychophysiological measures advantageous as tools for lie detection.

Another disadvantage of using psychophysiological measures in lie detection is that often, a psychophysiological relationship may be established with a particular measure when in fact this relation is not concomitant. As explained by Cacioppo et al. (2007), a psychophysiological concomitant is where there exists a many-to-one relationship such that a psychophysiological measure correlates with a known psychological state. However, it can be argued that changes in skin conductance thought to manifest when a person is lying may in fact turn out to be non concomitant. Cacioppo et al. indicate that manipulation of the same psychological element in a situation may alter or eliminate the covariation between the psychological and physiological elements because the latter is evoked not only by variations in the psychological element but also by variations in one or more additional factors. In a lie detection scenario, this would be the equivalent of skin conductance changes arising from an additional factor such as fear or anxiety such that the psychological state (guilt/innocence) may be overridden by this additional factor. So where a suspect is feeling anxious about being found guilty when they are in fact innocent, other factors may be at play in evoking the psychophysiological response and this may render the measured response non concomitant.

Psychophysiological science: interdisciplinary approaches to classic questions about the mind.(2007). In J. T Cacioppo, L.G. Tassinary & G.G. Berntson (Eds.), Handbook of psychophysiology, 3rd Edition. Cambridge, UK: Cambridge University Press.

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