# Overton Window Shift in the Dutch Parliament: Findings Report

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

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## 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.

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## 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](breakpoint_figure_1.png)

### 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.

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## 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](breakpoint_figure_2.png)

### 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).

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## 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](svd_drift_chart.png)

**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.

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## 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) |

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## 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.

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## 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)

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## 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.