Is there a natural boundary for formal thought disorder? Evidence from taxometric analysis

Abstract

Abstract Background Formal thought disorder (FTD) has been conceptualized as a categorical syndrome of psychosis, but this remains untested. Clarifying whether FTD is categorical or continuous and its supra-thres hold prevalence in non-clinical populations has direct implications for bears directly on mechanistic studies of FTD, psychosis prediction, and early intervention. Methods In a multi-site cross-sectional study of 520 individuals (352 clinical, 168 non-clinical), we used the Thought and Language Index to assess FTD across three subconstructs: Loosening, Peculiarities, and Impoverishment. Using complementary taxometric procedures (MAMBAC, MAXEIG, and L-Mode) with simulation of comparison data and multiple thresholding approaches (minimally important change criteria, ROC analysis, intuitive binarization), we evaluated the latent structure of FTD, optimal cutoff points to estimate FTD prevalence, and the frequency of co-occurrence of subconstructs. Results Taxometric analyses revealed a predominantly dimensional latent structure for FTD across various base rates. Among healthy individuals, Peculiarities were uncommon (1.2%), but 11.3% exhibited Loosening and 8.9% had Impoverishment even with conservative thresholds. Multi-construct co-occurrence exceeded chance across the sample but was restricted to patients at conservative clinical thresholds. Conclusions FTD exists in a continuous dimensional structure extending into healthy populations. Each dimension is measurable in healthy individuals, but multi-dimensional convergence at clinical thresholds (i.e. disorder with functional impairment) appears specific to patients. This suggests that common vulnerability factors increase the likelihood of elevations across multiple but substantially independent dimensions of FTD. As a hypothesis requiring prospective validation, clinical risk of psychosis may depend on the coalescence rather than single-dimensional severity of FTD subconstructs.