Deleterious effects of probe‐related versus irrelevant targets on the “CIT effect” in the P300‐ and RT‐based three‐stimulus protocol for detection of concealed information

Two groups of participants committed the same mock crime in which one of two items, a watch or a ring, was removed from a drawer and concealed. One group, the crime‐familiar group next experienced a three‐stimulus protocol (3SP), a Concealed Information Test (CIT), in which they were tested on the stolen (probe) item presented in a random series of five irrelevant (unseen) stimuli from the same jewelry category. A left‐hand button press, meaning “I don't recognize” was to follow each of these six items. A right‐hand press (“I do recognize”) was to follow the one other presented item, the target item, which in the case of the crime‐familiar group was the other, not‐stolen item in the drawer at the mock crime scene. For the other crime‐unfamiliar group, the target was a sixth unseen irrelevant item as in the original P300 CIT. In terms of P300 latency and reaction time (RT), crime‐familiar participants processed all stimuli faster than crime‐unfamiliar participants. The CIT effects (probe‐minus‐irrelevant differences) for crime‐familiar group members were inferior to those of crime‐unfamiliar group members for RT and P300 amplitude measures. Thus, familiar targets negatively impact the 3SP. This study examined for the first time the effect of incorporating meaningful target words related to the investigated guilty knowledge into the P300 brain wave‐based and reaction time‐based concealed information tests (CITs). It was found that such meaningful targets actually harm the detection accuracy of the CITs. Both types of CITs have been increasingly studied as potential aids for legal, judicial, and forensic practice. The CIT protocol is actually used in the field in Japan, and its use is being explored elsewhere. The present findings inform construction of these tests for research and field use.