Large Language Models (LLMs) are increasingly deployed in socially complex, interaction-driven tasks, yet their ability to mirror human behavior in emotionally and strategically complex contexts remains underexplored. This study assesses the behavioral alignment of personality-prompted LLMs in adversarial dispute resolution by simulating multi-turn conflict dialogues that incorporate negotiation. Each LLM is guided by a matched Five-Factor personality profile to control for individual variation and enhance realism. We evaluate alignment across three dimensions: linguistic style, emotional expression (e.g., anger dynamics), and strategic behavior. GPT-4.1 achieves the closest alignment with humans in linguistic style and emotional dynamics, while Claude-3.7-Sonnet best reflects strategic behavior. Nonetheless, substantial alignment gaps persist. Our findings establish a benchmark for alignment between LLMs and humans in socially complex interactions, underscoring both the promise and the limitations of personality conditioning in dialogue modeling.

Entrainment, the responsive communication between interacting individuals, is a crucial process in building a strong relationship between a mental health therapist and their client, leading to positive therapeutic outcomes. However, so far entrainment has not been investigated as a measure of efficacy of large language models (LLMs) delivering mental health therapy. In this work, we evaluate the linguistic entrainment of an LLM (ChatGPT 3.5-turbo) in a mental health dialog setting. We first validate computational measures of linguistic entrainment with two measures of the quality of client self-disclosures: intimacy and engagement (p < 0.05). We then compare the linguistic entrainment of the LLM to trained therapists and non-expert online peer supporters in a cognitive behavioral therapy (CBT) setting. We show that the LLM is outperformed by humans with respect to linguistic entrainment (p < 0.001). These results support the need to be cautious in using LLMs out-of-the-box for mental health applications.