No Free Lunch: Fundamental Limits of Learning Non-Hallucinating Generative Models

Changlong Wu, Ananth Grama, Wojciech Szpankowski

Keywords: generative models, hallucination, no-free-lunch theorem, distribution PAC learning, VC-dimension

TL;DR: We explore hallucination in generative models from a distribution PAC learning perspective, we prove both possibility and impossibility results on learnability across various paradigms.

Abstract: Generative models have shown impressive capabilities in synthesizing high-quality outputs across various domains. However, a persistent challenge is the occurrence of "hallucinations," where the model produces outputs that are not grounded in the underlying facts. While empirical strategies have been explored to mitigate this issue, a rigorous theoretical understanding remains elusive. In this paper, we develop a theoretical framework to analyze the *learnability* of non-hallucinating generative models from a learning-theoretic perspective. Our results reveal that non-hallucinating learning is statistically *impossible* when relying solely on the training dataset, even for a hypothesis class of size two and when the entire training set is truthful. To overcome these limitations, we show that incorporating *inductive biases* aligned with the actual facts into the learning process is essential. We provide a systematic approach to achieve this by restricting the fact set to a concept class of finite VC-dimension and demonstrate its effectiveness under various learning paradigms. Although our findings are primarily conceptual, they represent a first step towards a principled approach to addressing hallucinations in learning generative models.

Primary Area: learning theory

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Submission Number: 3203