Toward personalized psychotherapy: The importance of the trait-like/state-like distinction for understanding therapeutic change.

For the past hundred years, mechanisms of change have been the black box of psychotherapy. Thousands of studies failed to produce consistent findings, even concerning factors considered crucial for treatment success by theoretical models and decades of clinical experience. This article introduces the distinction between trait-like (TL) and state-like (SL) components of any mechanism of change (the TLSL distinction) as a potential key to the black box of psychotherapy. TL refers to individual differences between patients; SL refers to changes occurring within the patient over the course of treatment. The TLSL distinction explains why past research, which conflated the two, has produced conflicting results, and predicts the conditions under which consistent results can be obtained. Data collected so far show support for the importance of the TLSL distinction and point the way toward personalized treatment. The TL components create the individual's signature pathology and strengths map, and determine the SL changes that represent the patient-specific mechanisms most critical for optimizing treatment efficacy for each individual. The TLSL distinction has the potential to explain not only how psychotherapy works, but also how changes of any type occur in the wake of intervention, life events, and other factors. (PsycInfo Database Record (c) 2021 APA, all rights reserved).