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motief/reports/overton_window/findings_report.md

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Overton Window Shift in the Dutch Parliament: Findings Report

Date: 2026-05-08 Branch: feat/right-wing-motion-analysis Analysis period: 2016–2026

1. Summary

We tested the hypothesis that the Overton window shifted rightward in the Tweede Kamer using three indicators: centrist support for right-wing motions, content extremity trends, and SVD spatial drift. The strongest evidence is for centrist acceptance: support for right-wing motions surged post-2024 (d=+0.68), and the effect is even larger for opposition-only motions (d=+0.85) — ruling out a pure coalition explanation. However, content extremity did not increase (d=-0.09), and SVD axes proved too unstable for cross-window comparison. The shift is centered on the migration domain.

2. Indicator 1: Centrist Support Breakpoint

Core finding

Centrist support for right-wing motions rose from a pre-2024 mean of 0.384 to a post-2024 mean of 0.618 — a Cohen's d of +0.68 (medium-large effect). This is not a coalition artifact: opposition-only right-wing motions show an even larger increase, from 0.270 to 0.543 (d=+0.85, large effect).

Figure 1: Centrist Support and Pass Rate

Pass rate is an insensitive measure

Pass rates are near ceiling in all periods (96%+). In the Dutch parliament, nearly all motions pass regardless of content or political alignment. The plan's motivating concern about pass rate shifts (33% → 70%) was based on a different operationalization than what the data supports. With 96%+ passage rates, pass rate cannot serve as a shift indicator.

Domain decomposition

The shift is heavily migration-centric:

Domain Pre-2024 CS Post-2024 CS Δ
Migration 0.303 0.536 +0.233
Non-migration 0.529 0.605 +0.076

Migration is the primary vehicle for the observed shift. Non-migration right-wing motions already had moderate centrist support pre-2024, limiting room for growth.

Extremity-stratified tolerance test: Inconclusive

We tested whether centrists became more tolerant of high-extremity content specifically by bucketing motions by extremity score (1-2 mild, 2-3 moderate, 3-4 high, 4-5 extreme) and comparing pre/post pass rates. The test is underpowered: all buckets show 95-100% pass rates in both periods. With ceiling-level pass rates, there is no room to detect differential tolerance shifts.

3. Indicator 2: Content Extremity Trend

Core finding

Content extremity of right-wing motions did not increase (pre-2024: 2.21, post-2024: 2.15, d=-0.09). The Overton window shift is about acceptance of existing content — motions that were once beyond the pale are now supportable — not about increasingly radical proposals.

Figure 2: Extremity Trends and Stratified Pass Rate

LLM scoring reliability

A stratified manual audit of 20 motions (5 per extremity bucket) achieved 75% agreement (15/20), above the 70% threshold but borderline. Identified biases:

  • Anti-institutional overrating: LLM inflates scores on anti-EU and anti-government motions (procedural stances scored as radical policy)
  • Migration/cultural adjacency inflation: Motions mentioning migration-adjacent topics score higher than warranted
  • Climate topic inflation: Technical environmental motions scored higher than warranted

The LLM conflates stylistic extremity (inflammatory keywords, charged topics) with material impact (substantive rights restrictions, institutional change). This affects ~25% of scored motions, most pronounced in the high and extreme buckets.

Implication: Our content extremity measure is noisy. It captures a mix of stylistic and substantive radicalism. This is a known limitation documented in the plan's deferred follow-up work (two-dimensional scoring validation).

4. Indicator 3: SVD Spatial Drift — INCONCLUSIVE

Stability gate: FAILED

SVD axes were validated for stability across annual windows using Spearman rank correlation of party positions. 9 of 10 consecutive window pairs failed the ρ ≥ 0.7 threshold (maximum allowed: 2). Mean axis-1 correlation: ρ=0.0054; mean axis-2 correlation: ρ=0.2128.

This is the expected behavior of per-window SVD: principal axes are determined independently each year and have no inherent longitudinal alignment. Positions may reflect axis re-orientation rather than genuine ideological drift.

SVD Drift Chart

We cannot draw conclusions about spatial drift from SVD first-two-dimensions data. See the stability report for per-pair details.

Path forward

The explorer UI uses Procrustes-aligned PCA positions (load_party_scores_all_windows_aligned in analysis/explorer_data.py) which provide a common reference frame for cross-window comparison. A revised U3 could use this approach. However, we recommend against re-running U3 — the two strong indicators (centrist support surge, no extremity increase) already provide a clear picture, and adding spatial evidence would not change the qualitative conclusion.

5. Synthesis

What we can say with confidence

  1. Centrist parties are more willing to support right-wing motions post-2024 than before, and this is not explained by coalition membership. Cohen's d = +0.85 for opposition-only motions represents a large effect.
  2. The shift is migration-centric. Migration motions saw +0.233 centrist support gain; non-migration saw only +0.076. Migration is also the highest-extremity category and the only consistently negative-sentiment category.
  3. Content extremity did not increase. The window widened — what is acceptable grew — but the content of proposed motions is not more radical than before.

What we cannot say

  1. We cannot claim spatial (SVD) drift. Axes are too unstable for cross-window comparison.
  2. We cannot quantify how much of the shift is topic-driven vs. ideology-driven. Migration is inherently more controversial than other policy domains. If the volume of migration motions increased post-2024, centrist support for the category may reflect the topic's higher baseline controversy rather than shifting ideology.
  3. We cannot distinguish between sincere ideological shift and strategic adjustment. Centrist parties may genuinely agree more with right-wing content, or they may be voting differently for coalition-management or electoral reasons.

Uncertainty hierarchy

Evidence Level Indicator Status
Strong Centrist support surge (opposition-controlled) Confirmed
Moderate Migration-specificity of the shift Confirmed
Inconclusive Extremity-stratified tolerance shift Underpowered (pass rate ceiling)
Inconclusive SVD spatial drift Axes unstable
Weak Content extremity trend No increase (but LLM scoring imperfect)

6. Limitations

  • Small-N time series: 8 pre-2024 years, 3 post-2024 years (2026 is partial). Effect sizes are descriptive, not confirmatory.
  • LLM extremity scores: 75% audit agreement; borderline. Scores conflate stylistic and substantive radicalism. See deferred follow-up work for two-dimensional rescoring plan.
  • SVD axis instability: Cross-window SVD comparison is invalid without alignment. Spatial indicator discarded.
  • Coalition composition: Hardcoded per year. 2024 is ambiguous (Rutte IV until July, Schoof thereafter). Early 2024 motions may be miscoded.
  • Submitter party identification: Parsed from motion title prefixes. ~10% of motions have non-standard titles (bills, amendments) and are excluded from opposition-only analysis.
  • Pass rate baseline: Computed across motions with recorded votes. Unanimous consent motions are excluded, potentially biasing baseline upward. The Dutch parliament's near-universal passage rate makes pass rate a poor sensitivity measure.

7. Figures

  1. breakpoint_figure_1.png — Centrist support and pass rate over time (all RW, opposition-only, migration, non-migration, + baseline)
  2. breakpoint_figure_2.png — Extremity trends and extremity-stratified pass rate (pre vs. post 2024)
  3. svd_drift_chart.png — SVD centrist center trajectory (unreliable — axes unstable)

8. Next Steps

  1. Commit findings and archive the analysis on feat/right-wing-motion-analysis.
  2. Two-dimensional extremity rescoring (deferred): Validate whether LLM scores capture stylistic vs. material radicalism on a stratified sample. If correlation is low, rescore all motions with a refined dual-dimension prompt.
  3. Procrustes-aligned SVD (optional): If spatial evidence is desired, rerun U3 using load_party_scores_all_windows_aligned from explorer_data.py for a common reference frame.
  4. Temporal decomposition of migration vs. other domains: The 2024 shift may be partially explained by the increased volume of migration motions, rather than a general rightward shift. Disentangle topic composition from ideological drift.