Does suggestive information cause a confirmation bias in bullet comparisons?

Abstract 

Several researchers have argued that the confirmation bias, the tendency to selectively gather and process information such that it fits existing beliefs, is a main threat to objective forensic examinations. The goal of the present study was to empirically investigate whether examiners making bullet comparisons are indeed vulnerable to this bias. In the first experiment, six qualified examiners evaluated 6 sets of bullets that were presented to them twice. In the neutral task condition it was mentioned in the case description that there were two perpetrators and two crime scenes, whereas in the potentially biasing task condition it was mentioned that there was only one perpetrator and one crime scene. The results showed no effect of biased information on the decision outcome. An exploratory analysis revealed rather large individual differences in two cases. In a second study we compared the conclusions of first and second examiners of actual cases that were conducted in the period between 1997 and 2006. As the second examiner mostly has no context information it may be expected that the conclusion of the first examiner should be more extreme when he or she would have become prey to a confirmation bias. The results indicate an effect in the opposite direction: the first examiner gave less extreme ratings than the second one. In all, our results indicate that examiners were not affected by biased information the case description.

Keywords: Forensics, Observer effects, Bullet comparisons, Expert opinion