The Frequency Confound in Language-Model Surprisal and Metaphor Novelty
Summary
Researchers often treat language-model surprisal as a measure of how predictable a word is in context, and earlier work found that it tracks human judgments of metaphor novelty. This paper asks whether that link is driven by plain lexical frequency instead. The authors use the novelty ratings that Do Dinh et al. (2018) collected for metaphoric content words in VUAMC. They compare these ratings with two predictors:
- word-level surprisal from all eight Pythia sizes and from the 154 training checkpoints of Pythia-70M;
- two negative log frequency measures: NLF-Human, taken from the wordfreq library, and NLF-LM, counted in the Pythia pretraining tokens seen up to each checkpoint.
Frequency predicts novelty better than surprisal in every setting. Surprisal comes closest to novelty in small models at an early training stage, and these are also the conditions where surprisal most closely mirrors frequency. The authors conclude that the "optimal" small or early models reported in the literature may mostly be capturing frequency effects. They argue this should not be read as evidence for predictability-based accounts of metaphor novelty or of processing difficulty.
Key points
- Data: 15,155 metaphoric content words from VUAMC, with novelty scores between -1 and +1. Of these, 353 count as novel when the cut-off is set at 0.5.
- Frequency vs. novelty: Frequency outperforms surprisal throughout.
- NLF-Human: ρ = .66, AUC = .90
- NLF-LM: ρ = .63, AUC = .90
- Training dynamics in Pythia-70M: The surprisal–novelty association peaks after 128 steps (about 268M tokens), at ρ = .62. It then settles near ρ = .45 and AUC = .83.
- Surprisal vs. frequency: At that same peak, surprisal correlates almost perfectly with frequency (ρ = .95 with NLF-LM, .89 with NLF-Human). This later drops to around .60. Across model sizes the correlation ranges from .40 to .61 and decreases as models grow, matching the inverse scaling pattern seen for novelty.
- Limitation: The checkpoint analysis covers only Pythia-70M. Code and data are available at github.com/OmarMomen14/surprisal_frequency_novelty.
Cite
@inproceedings{momen2026frequency,
title = {{The Frequency Confound in Language-Model Surprisal and Metaphor Novelty}},
author = {Momen, Omar and Zarrieß, Sina},
booktitle = {Proceedings of the 15th Joint Conference on Lexical and Computational Semantics (*SEM 2026)},
year = {2026},
doi = {10.18653/v1/2026.starsem-conference.31},
url = {https://doi.org/10.18653/v1/2026.starsem-conference.31}
}References
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