[2505.16448] The First Impression Problem: Internal Bias Triggers Overthinking in Reasoning Models

Computer Science > Artificial Intelligence arXiv:2505.16448 (cs) [Submitted on 22 May 2025 (v1), last revised 1 Mar 2026 (this version, v4)] Title:The First Impression Problem: Internal Bias Triggers Overthinking in Reasoning Models Authors:Renfei Dang, Zhening Li, Shujian Huang, Jiajun Chen View a PDF of the paper titled The First Impression Problem: Internal Bias Triggers Overthinking in Reasoning Models, by Renfei Dang and 3 other authors View PDF Abstract:Reasoning models often exhibit overthinking, characterized by redundant reasoning steps. We identify \emph{internal bias} elicited by the input question as a key trigger of such behavior. Upon encountering a problem, the model immediately forms a preliminary guess about the answer, which we term an internal bias since it may not be explicitly generated, and it arises without systematic reasoning. When this guess conflicts with its subsequent reasoning, the model tends to engage in excessive reflection, resulting in wasted computation. We validate the association between internal bias and overthinking across multiple models and diverse reasoning tasks. To demonstrate the causal relationship more rigorously, we conduct two counterfactual interventions, showing that removing the input question after the model reduces the redundant reasoning across various complex reasoning tasks, and manually injecting bias affects overthinking accordingly. Further interpretability experiments suggest that excessive attention to the inpu...