Abstract
The use of generative large language models (LLMs) in mental health applications is gaining traction, with some proposals even suggesting LLM-based automated therapists. In this study, we assess the impact of fine-tuning therapist LLMs to improve the quality of therapy sessions, addressing a critical question in LLM-based mental health research. Specifically, we demonstrate that fine-tuning with datasets focused on specific therapeutic techniques significantly enhances the performance of LLM therapists. To facilitate this assessment, we introduce a novel evaluation system based on digital patients, powered by LLMs, which engage in text-based therapy sessions and provide session evaluations through questionnaires designed for human patients. This method addresses the inadequacies of traditional text-similarity metrics, which are insufficient for assessing the quality of therapeutic interactions. This study centers on motivational interviewing (MI), a structured and goal-oriented therapeutic approach. However, our digital therapists and patients can be adapted to work in other forms of therapy. We believe that our digital therapists offer a standardized method for assessing automated therapists and showcasing the potential of LLMs in mental health care.