LLMs are (still) mostly powered by imitative learning, not RL

Reinforcement learning from verifiable rewards (RLVR) is the hot new thing in LLM training. It’s so hot, and people spend so much time talking about it, that they sometimes lose sight of the big picture. Stepping back, LLMs can do lots of very impressive things. How? Where did those capabilities come from? Fundamentally, they come from a combination of: (1) Imitative learning , including pretraining and supervised fine-tuning (SFT) See my earlier discussion: “LLM pretraining magically transmutes observations into behavior, in a way that is profoundly disanalogous to how brains work” . (2) Reinforcement learning , including RL from human feedback [RLHF], RL from AI feedback [RLAIF], and especially RLVR. [1] If we look at the final trained LLM, we can ask how important each of those two pieces was, in explaining the LLM’s capabilities. And my claim is that it’s way more (1) than (2) . I'll start in §1 with some relevant evidence, and then in §2 I’ll circle back to operationalizing exactly what I’m claiming, and finally in §3, three reasons why we should care—namely, it affects how we should think about chain-of-thought legibility, about LLM capabilities, and about LLM alignment. Note that I am not arguing that RLVR does not importantly contribute to LLM capabilities. That would be absurd! Of course it does! Companies use RLVR because it works, and I expect them to continue doing so more and more. Again, the things I’m actually claiming are in §2–§3. 1. Some relevant evidence 1…