A controlled sandbox for studying how models acquire knowledge

Modern LMs are trained on everything at once, so it is hard to tell whether a new skill was learned or merely elicited. We constrain the training distribution itself: an 88B-token corpus filtered to the U.S. elementary-school curriculum, with models trained from scratch on it and matched unfiltered controls.

LittleCurriculum

An 88B-token corpus distilled from FineWeb-Edu through a five-stage filtering pipeline aligned with Common Core standards (K–5). Concepts, facts, and vocabulary taught above Grade 5 are explicitly excluded.

LittleLearner

Three scales (0.6B / 1.3B / 5B) trained from scratch on LittleCurriculum: chattable models with an interpretable knowledge boundary. Each ships with a matched Unfiltered control for clean comparison.

Elicitation, not acquisition

In our experiments, scaling, SFT+GRPO post-training, and in-context learning amplify what the curriculum taught, but none meaningfully improves out-of-scope performance, indicating that the pretraining filter sets the effective capability ceiling.

Model checkpoints

LittleLearner at three scales (0.6B / 1.3B / 5B), each with a matched Unfiltered control sharing its architecture, tokens, and recipe.

Base: the pretrained model. GRPO: math specialists post-trained on MathCAMPS; responses may exhibit a tendency toward math-oriented output. Chatty: variants tuned for general chat behavior.

Capability stays inside the curriculum

Can standard interventions push a model past what its pretraining data taught it? With the boundary under experimental control, we can ask cleanly. In our experiments, each intervention amplifies in-scope ability; none of them meaningfully improves out-of-scope performance.

Scaling

Scaling model size improves performance within the model’s controlled knowledge exposure and extends modestly to problems along the same learning trajectory, but yields little improvement on problems requiring more advanced capabilities outside the exposure.

Post-training

Post-training through GRPO significantly boosts in-scope K–5 capabilities, but fails to recover out-of-scope beyond-K–5 capabilities, even when training with out-of-scope data.

In-context learning

In-context learning with the prompts we test does not unlock new reasoning capabilities in beyond-K–5 for our trained 5B LittleLearner.

What will you teach it?

Because LittleLearner’s training exposure is explicitly specified, behavioral and representational changes can be related directly to the concepts you introduce. Three directions we’re excited about:

Can RL create capability?

The prior is restricted to K–5, so capabilities that emerge under RL can be attributed to the RL process itself. A tractable proxy for reward-driven discovery.

Watch a concept being learned

Introduce negative numbers and measure sample efficiency, retention, and interference. Or probe behavior near the boundary: does it answer, abstain, or hallucinate?

Machine vs. child learners

Specified exposure enables controlled human-model comparison. Do models and children need similar exposure to learn fractions, or make similar errors on word problems?

Bring your own question

A known boundary turns your idea into a clean experiment!

If you find this work useful

Please cite our paper:

@misc{littlelearner2026,
      title={LittleLearner: Language Models Under Pedagogically-Controlled Knowledge Exposure},
      author={Fanfei Li and Jana Zeller and Manuel Prada-Corral and Thaddäus Wiedemer and Prasanna Mayilvahanan and Ryan Cotterell and Wieland Brendel},
      year={2026},
      eprint={2608.13545},
      archivePrefix={arXiv},
      primaryClass={cs.CL},
      url={https://arxiv.org/abs/2608.13545}
}