Spurios Rewards
This work challenges prevailing assumptions about reinforcement learning with verifiable rewards (RLVR) in mathematical reasoning tasks. The authors show that Qwen2.5-Math models can improve significantly under RL, even when trained with spurious or flawed rewards.
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Surprisingly effective spurious rewards: The Qwen2.5-Math-7B model gains +21.4% accuracy with random rewards, +16.4% with format-based rewards, and +24.6% when explicitly trained on incorrect answers. These are close to the +28.8% gain from ground-truth reward signals, suggesting that RLVR surfaces latent capabilities rather than teaching new reasoning skills.
Model-specific generalization: Spurious rewards fail on other models like Llama3 or OLMo2. Only Qwen models consistently benefit, which the authors attribute to differences in pretraining. Notably, Qwen2.5-Math exhibits a unique “code reasoning” behavior, generating Python-like code to solve problems, which becomes more frequent post-RLVR and correlates strongly with accuracy.
Mechanism behind gains: The authors trace performance improvements to a shift in reasoning strategies. Most of the gain comes from language→code transitions, where the model switches from natural language to code reasoning during RLVR. Interventions that explicitly increase code usage (e.g., rewarding code-like responses or using a code-forcing prompt) boost performance further, but only on Qwen models.
Clipping bias enables learning from noise: Even with random rewards, performance improves due to GRPO’s clipping mechanism, which biases training toward reinforcing the model’s high-probability behaviors. These behaviors (e.g., code reasoning) happen to align with correctness in Qwen models but not in others.