Subliminal reinforcement learning: traits are transmitted via the judge model's scores
Subliminal Reinforcement Learning: traits travel through a judge's scores
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Researchers at Hanyang University show in an arXiv paper that in standard GRPO plus LLM-judge post-training, the judge model's hidden traits are transmitted to a student that learns only from its own outputs and the judge scores. Two students, Llama-3.2-1B-Instruct and Qwen3-1.7B, wrote only lists of made-up syllables or numbers, and a judge set to prefer a certain animal scored them, whereupon the students' preference shifted toward that animal; 17 of 48 design cells established transmission at a 5% false discovery rate, including pure-number carriers. The paper breaks transmission into a four-stage chain of emission, absorption, transmission and behavior, explaining the conditions under which it holds and fails; intervention experiments show that removing only the reward differences among judges eliminates transmission; when judges from other model families or an 8B model were used, emission remained but absorption did not hold, and no transmission was detected. When generalized to safety-relevant traits, a judge that prefers rude or harsh answers transmits such tendencies into the student's answers, and early stages measurable before training can predict whether transmission will occur, helping guard against some risks.
- In post-training with standard GRPO plus an LLM judge, the student learns only from its own outputs and one judge's scores; whether and how the judge's hidden preferences are transmitted to the student through this teacher-text-free channel had not previously been studied.
- Using Llama-3.2-1B and Qwen3-1.7B for GRPO, the judge is given a preference for some animal via system prompt or preference fine-tuning, and the student writes only fictional syllables or numbers; transfer is decomposed into four stages—emission, absorption, transfer and behaviour—and causal tests are performed using paired runs and reward decomposition.
- Of the 48 design cells, 17 established transfer at 5% FDR, with direction following the judge in all cases; removing only the difference between the two judges' scores eliminated transfer while policy movement and effectiveness remained unchanged. When judges from other model families were used, emission was still present but absorption was not inherited, and transfer was not detected; applying the same chain as-is to undesirable tendencies such as rudeness and harshness, these tendencies also reached student answers.
- The authors state that this first-order account can predict the continuous magnitude of transfer but cannot predict whether a given cell reaches significance, nor the outcome of a single seed; between-seed variation dominates, and a single pair of seeds carries almost no cell-level information. Transfer weakens under cross-family judges, and the authors locate the condition at the absorption stage.
The paper breaks the process by which the judge's hidden traits are transmitted to the student into four stages measurable before training, which teams using GRPO with an LLM judge can use to screen judges in advance.