Research evidence suggests Therapeutic Assessment positively affects clients with problems in living, including clients with personality disorders, who are typically quite resistant to change. Importantly, this change takes place quickly, in relatively few sessions. This article draws on a relatively new evolutionary-based theory of epistemic trust (ET) and epistemic hypervigilance (EH) as a lens to plausibly explain the efficacy of TA, and especially its influence on PD clients' alliance and motivation for subsequent psychotherapy (Fonagy, Luyten, & Alison, 2015). ET is the willingness to take in relevant interpersonally transmited information and it is essential to the immediate success of psychotherapy and its long-term impact. The collaborative, intersubjective framework of TA and many of its specific techniques might be understood as highly relevant to restoring ET in clients, especially those with PD. We close by discussing implications for psychological assessment, psychotherapy, and research.