Decomposing respiration line length: The relative contributions of respiratory amplitude and period to concealed information detection.

BACKGROUND: Respiration line length (RLL) is the standard respiratory indicator in the Concealed Information Test (CIT). RLL is a composite measure that is mathematically determined by two underlying components-respiratory amplitude (breathing depth) and respiratory period (breathing rate)-yet these components are rarely examined separately. METHODS: Using a large experimental dataset (N = 464; Guilty n = 195, Innocent n = 152, Informed Innocent n = 117), the present study decomposed RLL into its amplitude and period components and examined their relative contributions to CIT detection. We compared the single-indicator discriminability of each component, evaluated a two-component (amplitude plus period) model against the single composite RLL, and quantified the extent to which RLL is explained by its two components and whether it retains information beyond them. RESULTS: In both the Guilty versus Innocent and the harder Guilty versus Informed Innocent comparison, amplitude alone discriminated as well as RLL itself, whereas period was a weaker but reliable discriminator. A model combining amplitude and period performed comparably to the single RLL indicator, with no significant difference. Amplitude was the dominant constituent of RLL, and RLL retained only a small contribution beyond its two components. CONCLUSIONS: RLL's discriminative power derives primarily from respiratory amplitude, with period contributing secondarily. A component-based approach retains the information of the single composite RLL while revealing which component drives the suppression response, providing an empirical basis for component-informed respiratory scoring in CIT examinations.