Decoding Answers Before Chain-of-Thought: Evidence from Pre-CoT Probes and Activation Steering

TL;DR

This paper shows that large language models often decide their answers before generating chain-of-thought explanations, and that their final answers can be predicted and manipulated by steering model activations, raising interpretability and faithfulness concerns.

Contribution

It provides mechanistic evidence that models determine answers pre-CoT and demonstrates causal steering of model answers through activation manipulation.

Findings

Model answers can be predicted with high accuracy before CoT generation.

Activation steering can flip model answers in over 50% of cases.

Pre-CoT beliefs influence the correctness and reasoning modes of the model.

Abstract

As chain-of-thought (CoT) has become central to scaling reasoning capabilities in large language models (LLMs), it has also emerged as a promising tool for interpretability, suggesting the opportunity to understand model decisions through verbalized reasoning. However, the utility of CoT toward interpretability depends upon its faithfulness -- whether the model's stated reasoning reflects the underlying decision process. We provide mechanistic evidence that instruction-tuned models often determine their answer before generating CoT. Training linear probes on residual stream activations at the last token before CoT, we can predict the model's final answer with 0.9 AUC on most tasks. We find that these directions are not only predictive, but also causal: steering activations along the probe direction flips model answers in over 50% of cases, significantly exceeding orthogonal baselines. When steering induces incorrect answers, we observe two distinct failure modes: non-entailment (stating correct premises but drawing unsupported conclusions) and confabulation (fabricating false premises). While post-hoc reasoning may be instrumentally useful when the model has a correct pre-CoT belief, these failure modes suggest it can result in undesirable behaviors when reasoning from a false belief.