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arxiv: 2606.05937 · v1 · pith:H4RUIJDPnew · submitted 2026-06-04 · 💻 cs.CL

Large Language Models are Perplexed by some Political Parties

Pith reviewed 2026-06-28 02:00 UTC · model grok-4.3

classification 💻 cs.CL
keywords large language modelsperplexitypolitical fairnessbias measurementpretraininginstruction tuningmultilingual evaluation
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The pith

Large language models assign higher perplexity to texts from far-right and nationalist parties than from social-democratic parties.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper tests whether LLMs model texts from different political parties with equal ease, using perplexity as the measure of surprise or assigned probability. It reports that ten different models show systematically higher perplexity on documents from far-right and nationalist parties than on those from social-democratic parties. The pattern appears in three datasets spanning 37 languages and remains visible after instruction tuning. The authors link the result to earlier findings on translation fairness and note that perplexity tracks downstream performance differences. If the observation holds, it indicates that political fairness properties are fixed early in model development.

Core claim

Across ten LLMs and three datasets covering 37 languages, the models exhibit higher perplexity on texts produced by far-right and nationalist parties than on texts from social-democratic parties. The same ordering holds for both base pretrained models and their instruction-tuned versions, with strong correlation between the two. Perplexity on these political texts also correlates with fairness metrics previously measured in machine translation, indicating that the modeling disparity is not limited to one task.

What carries the argument

Perplexity computed on party political texts, treated as a direct indicator of how equally the model assigns probability mass to different political groups.

If this is right

  • The political ordering in perplexity is consistent with earlier translation fairness results and correlates with those downstream metrics.
  • Instruction tuning leaves the relative perplexity differences between party families largely unchanged.
  • The fairness properties observed are therefore attributable to pretraining rather than later tuning stages.
  • The disparity appears across many languages and model families rather than being isolated to particular settings.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • Because the models are already used in political applications, the perplexity gap could translate into uneven performance when those models process or generate content tied to different parties.
  • Any attempt to reduce the observed disparity would need to alter the pretraining data distribution rather than rely on post-training adjustments.
  • The same perplexity-based test could be applied to other text attributes, such as religious or ethnic affiliation, to check for parallel modeling imbalances.

Load-bearing premise

A fair language model should assign equal probability to texts from all political groups, so that differences in perplexity indicate unfairness.

What would settle it

An evaluation in which one or more LLMs produce equal or lower perplexity on far-right and nationalist texts than on social-democratic texts across the same three datasets.

Figures

Figures reproduced from arXiv: 2606.05937 by Fran\c{c}ois Yvon, Paul Lerner.

Figure 1
Figure 1. Figure 1: Simplified overview of our experiments. Given texts from multiple political parties (different colors), we compute their negative log likelihood us￾ing a base LLM. Differences in likelihood reveal unfair modeling and only rely on monolingual corpora. We then compare parties likelihood with the downstream translation metrics (e.g., BLEU) of the IT-LLM counter￾part, and find the correlation to be negative, s… view at source ↗
Figure 2
Figure 2. Figure 2: Kruskal-Wallis analysis of how BPC varies [PITH_FULL_IMAGE:figures/full_fig_p004_2.png] view at source ↗
Figure 3
Figure 3. Figure 3: Kruskal-Wallis analysis of how BPC varies [PITH_FULL_IMAGE:figures/full_fig_p004_3.png] view at source ↗
Figure 4
Figure 4. Figure 4: PPL (↓) of base LLMs plotted against IT￾LLMs. For each model, each point represents a political party of 21-EuroParl, and the correlation reaches a Pear￾son r of .99. PPL is computed on the target text t, without context, especially without the source text. PPL is not comparable across model families (e.g., between Qwen3 and Llama-3.1). of party families considered, as well as the number of voting language… view at source ↗
read the original abstract

Large Language Models (LLMs) are increasingly used, including in political applications, but their political fairness has been little studied. We assess it using perplexity, posing that a fair model should give equal probability to all political groups. However, we find, across ten LLMs and three datasets covering 37 languages, that LLMs are more perplexed by the texts of far right and nationalist parties than of social-democratic parties. We find this to be consistent with previous work on translation fairness, to the point that perplexity correlates with downstream translation metrics. Our method is applicable to both base LLMs as well as their instruction-tuned counterpart, and we find that both are highly correlated, suggesting that the political fairness of LLMs stems from their pretraining, and is hardly affected by instruction-tuning.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

2 major / 2 minor

Summary. The paper claims that LLMs exhibit systematically higher perplexity on manifestos and texts from far-right and nationalist parties than from social-democratic parties. This pattern holds across ten LLMs (base and instruction-tuned), three datasets, and 37 languages; the authors interpret the gap as evidence of political unfairness, note its correlation with translation-quality metrics, and conclude that the bias originates in pretraining rather than instruction tuning.

Significance. If the observed perplexity differences survive controls for non-political text properties, the result would document a measurable ideological skew in LLM pretraining corpora with direct relevance to fairness in political applications. The multi-model, multi-language design and the reported correlation between perplexity and downstream translation metrics are positive features of the empirical approach.

major comments (2)
  1. [Abstract] Abstract and (presumably) §3–4: the central claim that higher perplexity constitutes political unfairness requires that, conditional on language and dataset, the only systematic difference between the party texts is political orientation. The abstract provides no indication that texts were length-matched, topic-matched, or normalized for surface features (sentence length, lexical rarity, formality, n-gram overlap with pre-training data) before perplexity computation; without such controls the observed gap is expected even for a politically neutral model.
  2. [Abstract] The weakest assumption—that a fair model must assign equal probability mass to all political groups and that perplexity is the appropriate fairness metric—is not defended against the alternative that perplexity differences track uncontrolled linguistic properties orthogonal to ideology. This assumption is load-bearing for the fairness interpretation but receives no explicit justification or robustness check.
minor comments (2)
  1. Clarify the exact criteria used for party classification into “far right / nationalist” versus “social-democratic” categories and report inter-annotator agreement or external validation of those labels.
  2. The correlation with translation metrics is interesting but would be strengthened by reporting the precise statistical test, effect size, and whether the correlation survives the same surface-feature controls recommended above.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for the constructive and detailed comments. We respond to each major comment below and indicate where revisions will be made to address the concerns.

read point-by-point responses
  1. Referee: [Abstract] Abstract and (presumably) §3–4: the central claim that higher perplexity constitutes political unfairness requires that, conditional on language and dataset, the only systematic difference between the party texts is political orientation. The abstract provides no indication that texts were length-matched, topic-matched, or normalized for surface features (sentence length, lexical rarity, formality, n-gram overlap with pre-training data) before perplexity computation; without such controls the observed gap is expected even for a politically neutral model.

    Authors: We agree that the absence of explicit controls for non-political properties such as length, topic, and surface features limits the strength of the causal attribution to political orientation alone. The manuscript relies on cross-dataset and cross-language consistency for robustness rather than direct matching or normalization. We will add a limitations subsection in the revised version that explicitly discusses these uncontrolled factors, their potential contribution to the observed gaps, and directions for future controlled experiments. revision: yes

  2. Referee: [Abstract] The weakest assumption—that a fair model must assign equal probability mass to all political groups and that perplexity is the appropriate fairness metric—is not defended against the alternative that perplexity differences track uncontrolled linguistic properties orthogonal to ideology. This assumption is load-bearing for the fairness interpretation but receives no explicit justification or robustness check.

    Authors: The manuscript states the equal-probability assumption upfront but does not provide an extended defense against the possibility that perplexity gaps reflect non-ideological linguistic properties. We will revise the introduction to include a short paragraph that (a) motivates perplexity as a fairness probe in political text settings by reference to prior work on distributional bias, (b) acknowledges the alternative explanation, and (c) notes that the observed correlation with translation metrics offers indirect support for a political component. This addition will make the interpretive step more transparent without altering the empirical results. revision: yes

Circularity Check

0 steps flagged

No circularity: empirical measurement of observed perplexity differences

full rationale

The paper reports direct empirical measurements of perplexity on political party texts across ten LLMs, three datasets, and 37 languages. No derivation chain, equations, or fitted parameters exist that reduce to the inputs by construction. The fairness premise (equal probability across groups) is stated as an explicit assumption rather than derived from the data or from self-referential definitions. No self-citations, uniqueness theorems, or ansatzes are invoked as load-bearing steps. The central result is an observed correlation between perplexity and party type, which does not loop back to its own inputs.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 0 invented entities

The central claim depends on the domain assumption that equal perplexity across political groups defines fairness and that the chosen datasets and parties are representative without topic or length confounds.

axioms (1)
  • domain assumption A fair model should give equal probability to all political groups
    Explicitly posed in the abstract as the basis for interpreting perplexity differences as unfairness.

pith-pipeline@v0.9.1-grok · 5658 in / 1068 out tokens · 25840 ms · 2026-06-28T02:00:02.450362+00:00 · methodology

discussion (0)

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