Pre-Test Physiological Index Selection in the Concealed Information Test

The Concealed Information Test (CIT) is widely used in forensic contexts, particularly in Japan, to detect concealed knowledge through psychophysiological responses. Building on prior work on individual response specificity (IRS), this study examined whether pre-selecting diagnostic physiological indices based on performance in a preliminary demonstration (card) test influences the magnitude of subsequent CIT effects. Forty-six university students participated in a mock theft paradigm consisting of a card test and a watch trial. Skin conductance response (SCR), respiration speed (RS), and heart rate (HR) were recorded. Difference z-scores were used to index CIT effects. CIT effects were compared across three analytical patterns: using all three indices, using two indices excluding the weakest one, and using only the single most responsive index identified in the card test. Bayesian and frequentist repeated-measures analyses were conducted. Across both tasks, all three physiological indices showed positive CIT effects. Comparisons across index-selection conditions indicated differences in the magnitude of CIT effects. Results indicated that relying on a single, individually diagnostic index yielded numerically greater standardized within-subject differences between critical and noncritical items (i.e., larger CIT effects as indexed by difference z-scores) than combining multiple indices, although the associated Bayes factors and effect sizes were modest. Rather than focusing on detection accuracy, this study quantitatively examined differences in effect magnitude associated with index pre-selection using experimentally obtained within-participant data. The findings provide cautious support for IRS-informed index selection as a practical strategy for improving the interpretability of CIT outcomes.