The mechanism of filler items in the response time concealed information test

The response time concealed information test (RT-CIT) can reveal that a person recognizes a relevant (probe) item among other, irrelevant items, based on slower responding to the probe compared to the irrelevant items. Therefore, if this person is concealing the knowledge about the relevance of this item (e.g., recognizing it as a murder weapon), this deception can be unveiled. Adding familiarity-related filler items to the task has been shown to substantially increase the validity of the method, but assumptions for this effect have never been tested before. In the present series of three experiments (N = 511), we tested several factors, most of which were found to indeed influence the enhancing effects of fillers. First, larger enhancement is achieved when a smaller proportion of fillers shares the response key with the target. Second, familiarity context does play a role in the enhancement, and the target sharing its response key with the familiarity-referring fillers leads to larger enhancement. Third, mere symbolic fillers (such as simple arrow-like characters) also lead to enhancement, but filler words without task-relevant meaning are not effective. Fourth, small visual differences (lettercase or underlining) between fillers and the rest of the items have no significant influence. All this provides justification for the original structure of the fillers and also demonstrates that the enhancement is highly generalizable: Fillers have a potential to improve the RT-CIT regardless of deception scenario, item types, or the examinee's language comprehension.