Salience of guilty knowledge test items affects accuracy in realistic mock crimes

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Abstract

A Guilty Knowledge Test measuring electrodermal reactions was carried out in order to investigate the quality of different questions and the validity of the test in a situation that resembled a true crime. Fifty participants were randomly assigned to commit one of two realistic mock crimes, and were later tested with GKTs concerning both the crime they had enacted and the one they had no knowledge of. Different scoring systems (SCRs and peak amplitudes as well as raw and standardised scores) were employed and compared when analyzing the results. Although there were some false positives, the test was able to differentiate between the groups of guilty and innocent participants. With the best scoring systems, the test was able to classify up to 84% of the innocent and up to 76% of the guilty correctly according to a logistic regression analysis. ROC areas reflecting these same results reached values above .80. Questions on matters that demanded the participants' attention and were easier to remember had better discriminative power. With nearly all scoring methods, there was a significant interaction between the salience of the relevant items and the guilt of the participants. Participants reacted more strongly to salient relevant items when they were guilty, while no different reactions were observed for the non-salient items between guilty and innocent participants. It is suggested that, although the Guilty Knowledge Test appears to be a valid measure of guilty knowledge even in crimes that are close to real crimes, the principles on which guilty knowledge test questions are constructed should be more clearly specified.

Section snippets

Participants

Participants consisted of 30 men and 20 women. Their ages ranged from 20 to 38 (M = 25.5, S.D. = 4.2). The participants were students and academics who were recruited through e-mail lists. The participants received a movie ticket as a reward for their participation.

Experimental setting

The mock crimes were conducted in two different environments. The library of the Police College served as the first environment with minor modifications to its normal appearance. The library is quite small and consists of two rooms that

Preliminary analyses

In a correlation analysis, no relationship was found between the electrodermal reactions and the age or sex of the participants in either mock crime. Sex had also no effect on the accuracy of the test based on any of the scoring methods. When three outliers were removed from the analysis of the association between age and accuracy, only one correlation out of 26 was significant which can be attributed to chance. Neither age nor sex was analyzed further.

In order to make sure that the question

Discussion

The validity of the GKT was investigated using mock crimes that were more realistic than those usually employed in laboratory research. The GKT was able to differentiate guilty and innocent participants at a reasonable rate in two different mock crimes. This main result agrees with prior studies (Ben-Shakhar and Elaad, 2003, Carmel et al., 2003). What gives value to the study is that both guilty and innocent participants were included, unlike in some other studies with guilty participants only,

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