From Loops to Oops: Fallback Behaviors of Language Models Under Uncertainty

Maor Ivgi, Ori Yoran, Jonathan Berant, Mor Geva

Keywords: Hallucinations, Epistemic uncertainty, Degenerate text, Fallback behaviors

TL;DR: When uncertain, LLMs exhibit fallback behaviors like degenerate text and hallucinations. These behaviors are linked and follow a strict order. Factors like pretraining, parameter count, and instruction-following training affect which fallback is used

Abstract:

Large language models (LLMs) often exhibit undesirable behaviors, such as hallucinations and sequence repetitions.
We propose to view these behaviors as fallbacks that models exhibit under epistemic uncertainty, and investigate the connection between them.
We categorize fallback behaviors — sequence repetitions, degenerate text, and hallucinations — and extensively analyze them in models from the same family that differ by the amount of pretraining tokens, parameter count, or the inclusion of instruction-following training.
Our experiments reveal a clear and consistent ordering of fallback behaviors, across all these axes:
the more advanced an LLM is (i.e., trained on more tokens, has more parameters, or instruction-tuned),
its fallback behavior shifts from sequence repetitions, to degenerate text, and then to hallucinations.
Moreover, the same ordering is observed during the generation of a single sequence, even for the best-performing models; as uncertainty increases, models shift from generating hallucinations to producing degenerate text and finally sequence repetitions.
Lastly, we demonstrate that while common decoding techniques, such as random sampling, alleviate unwanted behaviors like sequence repetitions, they increase harder-to-detect hallucinations.

Primary Area: foundation or frontier models, including LLMs

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Submission Number: 2833