The valence of client and therapist language reflects symptom changes in messaging-based psychotherapy

Psychopathology is often characterized by intense negative emotions. Understanding how these emotions are expressed through language can provide valuable insights on emotional processing and reveal linguistic patterns associated with mental health conditions. Sentiment, or the emotional tone of language, has been linked to affective states and wellbeing, suggesting it could serve as a marker of psychopathology and therapeutic progress. However, the versatility of different approaches to measuring sentiment makes it unclear which methods are best, and it is unclear whether language sentiment reflects symptoms or precedes them in longitudinal therapeutic settings. This study examined whether changes in client sentiment, therapist sentiment, or their divergence tracked clients' internalizing symptoms using a large dataset of text-based psychotherapeutic exchanges. We compared how well 13 commonly used sentiment analysis measures predicted internalizing symptoms in exploratory (N=3,729) and validation (N=2,500) datasets of messaging-based therapeutic conversations. Both client and therapist sentiment became more positive over time, and more positive sentiment preceded improving symptoms. Diverse sentiment measures mostly behaved similarly, although approaches that estimate word-level sentiment and treat positivity and negativity as separate constructs tended to yield more consistent results particularly within-subjects. These findings suggest that language sentiment can be a marker of internalizing symptoms and therapeutic progress, highlighting the potential of sentiment analysis as a tool for monitoring treatment outcomes.