Why is it hard to assess thought disorder? Clarifying the third domain of psychosis.

Summary

Formal Thought Disorder (FTD) is strongly linked to poor outcomes and genetic risk in psychosis, yet it remains poorly defined and rarely measured in clinical practice. A systematic review of 50 years of assessment tools (16 rating scales, 32 factor analyses) reveals that research has treated FTD as a natural kind—a latent entity causing observable signs—but empirical evidence contradicts this. Construct definitions are radically heterogeneous, factor structures do not replicate at the item level, and no essential properties are universal. The authors propose measuring FTD as a Constituted Practical Entity: a probabilistic cluster of linguistic-cognitive features whose interaction produces communication failure, with no single feature necessary or sufficient. This framework reconciles FTD's multidimensional nature with measurement and recommends establishing consensus constituents, measuring their interactions, and developing computational tools.

Study at a glance

Abstract

Formal Thought Disorder (FTD) presents psychiatry's central paradox: it is one of the robust predictors of poor outcomes and polygenic risk in psychosis yet remains poorly defined and rarely measured clinically. We systematically reviewed 50 years of FTD assessment (16 rating scales, 32 factor analyses) to understand this paradox. Research to date has implicitly treated FTD as a natural kind, a latent entity that causes observable signs. Yet, empirical evidence contradicts this assumption: we find radical heterogeneity in construct definition, non-replicability of factor structures at the item level, and no universally essential properties across items. We propose measuring FTD as a Constituted Practical Entity: a probabilistic cluster of linguistic-cognitive features whose interaction produces communication failure. In this framework, no single feature is necessary or sufficient; dysfunction emerges from their relationships, not from a single latent process. This reconceptualization reconciles the multi-dimensional nature of FTD with our attempts to measure it and offers a clear research path: establish consensus constituents, measure their interactions, and develop computational tools. Without addressing the conceptual foundations, technical advances will only perpetuate existing confusions. Our framework clarifies what is being measured in the name of FTD and guides the development of computational tools and clinically meaningful targets for this third domain of psychosis.