People involved in criminal proceedings (e.g. police officers, district attorneys, judges, and jury members) may run the risk of developing confirmation bias, or tunnel vision. That is, these parties may readily become convinced that the suspect is guilty, and may then no longer be open to alternative scenarios in which the suspect is actually innocent. This may be reflected in a preference for guilt-confirming investigation endeavours, as opposed to investigations that are aimed at confirming, or even excluding, alternative scenarios. In three studies, participants read a case file, and were subsequently instructed to select additional police investigations. Some of these additional endeavours were guilt-confirming (i.e. incriminating), whereas others were disconfirming (i.e. exonerating). Results suggest that additional investigation search was guided by an initial assessment of the suspect's guilt (Study 1). Furthermore, participants' tendency to select incriminating investigations increased with increased crime severity, and with the strength of the evidence present in the case file. Finally, the selection of incriminating investigations was associated with conviction rates (Study 3). However, in general, participants did not favour incriminating endeavours. That is, in the three studies, the percentages of selected incriminating endeavours did hardly or not exceed 50%.

Confirmation bias, Criminal investigation, Decision making, Tunnel vision,
Journal of Investigative Psychology and Offender Profiling
Department of Psychology

Rassin, E.G.C, Eerland, A, & Kuijpers, I. (2010). Let's find the evidence: An analogue study of confirmation bias in criminal investigations. Journal of Investigative Psychology and Offender Profiling, 7(3), 231–246. doi:10.1002/jip.126