Title: Your Language Model Secretly Contains Personality Subnetworks
Primary area: foundation or frontier models, including LLMs
Keywords: Large Language Models, Persona Modeling

Abstract: Large Language Models (LLMs) demonstrate remarkable flexibility in adopting different personas and behaviors. Existing approaches typically adapt such behavior through external knowledge such as prompting, retrieval-augmented generation (RAG), or fine-tuning. We ask: do LLMs really need external context or parameters to adapting to different behaviors, or do they already have such knowledge embedded to their parameters?
In this work, we show that LLMs already contain persona-specialized subnetworks in their parameter space. Using small calibration datasets, we identify distinct activation signatures associated with different personas. Guided by these statistics, we develop masking strategy that isolate lightweight persona subnetworks. Building on the findings, we further discuss: how can we discover opposing sub-network from the model that lead to binary-opposing personas, such as introvert-extrovert? 
To further enhance separation in binary opposition scenarios, we introduce a contrastive pruning strategy that identifies parameters responsible for the statistical divergence between opposing personas. Our method is entirely training-free, and rely solely on the language model's existing parameter space. Across diverse evaluation settings, the resulting subnetworks exhibit significantly stronger persona alignment than baselines that requires external knowledge while being more efficient. Our findings suggest that diverse human-like behaviors are not merely induced in LLMs, but are already embedded in their parameter space—pointing toward a new perspective on controllable and interpretable personalization in large language models. Our code is available at  https://anonymous.4open.science/r/C694.