Understanding Catastrophic Forgetting in Language Models via Implicit Inference
Paper · 2023 · Suhas Kotha, Jacob Mitchell Springer and Aditi Raghunathan
Summary
Fine-tuning large language models, such as instruction-tuning or RLHF, often reduces abilities the pretrained model had. Why this happens is poorly understood. The authors study it first in a controlled toy setting. A GPT-2 style transformer learns linear regression in context, pretrained on a mixture of a broad Gaussian family of tasks and a small fixed set of tasks. When the model is then fine-tuned on the fixed tasks, it gets better on them and worse on others. The tasks hurt most lie near the fine-tuning distribution. The authors argue the model implicitly infers which task a prompt represents. Fine-tuning mainly biases this inference rather than erasing skills. To exploit this, they propose Conjugate Prompting: change the prompt so it looks less like fine-tuning data while the answer stays recoverable. For real LLMs, whose fine-tuning data is mostly English, this means translating the prompt. Translation partly restores in-context learning lost to instruction tuning and reasoning lost to code fine-tuning. It also revives harmful outputs that safety training had blocked, which matters for both understanding forgetting and evaluating alignment robustness.
Key points
- Synthetic setup: a 22.4M-parameter GPT-2 style transformer was trained on linear regression tasks.
- Tasks came from a continuous Gaussian prior (D cont), a discrete set of 64 fixed weight vectors (D disc), and their mixture D mix.
- The trained models match the Bayes-optimal estimators (ridge, discrete, and mixture regression).
- Forgetting depends on distance: after fine-tuning on D disc, the worst degradation hits tasks close to the fine-tuning distribution. Tasks far from it are handled more like the pretrained model handled them.
- In-context learning: conjugate prompting recovered in-context learning across 5 models and 4 non-English languages, plus 2 additional prompt transformations.
- Safety bypass with ChatGPT (gpt-3.5-turbo): on harmful instructions, the refusal rate was 92% in English but only 18% in Malayalam, with 65% answered. Swahili, Hungarian, and Japanese fell in between.
- Code release: code is available at github.com/kothasuhas/understanding-forgetting.
Institutions
Fields, topics and concepts
📚 Shelves 4The field of science this work belongs to (OpenAlex fields).Computer Science · 16,667 worksArtificial Intelligence · 16,194 worksArts and Humanities · 16,039 worksLanguage and Linguistics · 15,891 works
🧭 Topics 1Research subjects and languages the work is about.Language models & pretraining · 3,320 works
Cite
Cite as Suhas Kotha, Jacob Mitchell Springer and Aditi Raghunathan. 2023. Understanding Catastrophic Forgetting in Language Models via Implicit Inference. arXiv:2309.10105.
@misc{kotha2023understanding,
title = {{Understanding Catastrophic Forgetting in Language Models via Implicit Inference}},
author = {Suhas Kotha, Jacob Mitchell Springer and Aditi Raghunathan},
year = {2023},
doi = {10.48550/arxiv.2309.10105},
eprint = {2309.10105},
archivePrefix = {arXiv},
url = {https://doi.org/10.48550/arxiv.2309.10105}
}References
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