The impact of prior knowledge from participant instructions in a mock crime P300 Concealed Information Test

• We tested the effect of information exposure from instructions on a P300-CIT.
• Simple knowledge of crime details induced false-positives in innocent participants.
• Information exposure biased CITs toward enhanced sensitivity in guilty participants.

In P300-Concealed Information Tests used with mock crime scenarios, the amount of detail revealed to a participant prior to the commission of the mock crime can have a serious impact on a study's validity. We predicted that exposure to crime details through instructions would bias detection rates toward enhanced sensitivity. In a 2 × 2 factorial design, participants were either informed (through mock crime instructions) or naïve as to the identity of a to-be-stolen item, and then either committed (guilty) or did not commit (innocent) the crime. Results showed that prior knowledge of the stolen item was sufficient to cause 69% of innocent–informed participants to be incorrectly classified as guilty. Further, we found a trend toward enhanced detection rate for guilty–informed participants over guilty–naïve participants. Results suggest that revealing details to participants through instructions biases detection rates in the P300-CIT toward enhanced sensitivity.