Hacking Generative Perplexity: Why Unconditional Text Evaluation Needs Distributional Metrics
Current non-autoregressive language models commonly rely on generation perplexity (gen-PPL) to evaluate text quality; however, this metric fails to adequately capture grammatical correctness and semantic coherence. This work proposes a zero-parameter naive sampler that achieves state-of-the-art gen-PPL on LM1B and OpenWebText yet produces clearly incoherent text, thereby systematically exposing the fundamental limitations of gen-PPL for the first time. To address this issue, we introduce direct evaluation methods based on distributional divergences—such as KL and Jensen–Shannon divergence—and scoring from pretrained autoregressive models. Our experiments demonstrate that these distribution-based metrics provide a more faithful and effective assessment of generation quality in unconditional text generation, establishing their necessity and superiority over conventional gen-PPL.