Policy Diffusion, Popularity Cues, and Ideological Alignment: An RCT with Spanish Municipalities

Last registered on May 18, 2026
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Pre-Trial

Trial Information

General Information

Title
Policy Diffusion, Popularity Cues, and Ideological Alignment: An RCT with Spanish Municipalities
RCT ID
AEARCTR-0018667
Initial registration date
May 18, 2026

Initial registration date is when the trial was registered.

It corresponds to when the registration was submitted to the Registry to be reviewed for publication.

First published
May 18, 2026, 8:35 AM EDT

First published corresponds to when the trial was first made public on the Registry after being reviewed.

Locations

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Primary Investigator

Name
Francesco Capozza
Affiliation
UB, IEB
Contact Primary Investigator

Other Primary Investigator(s)

PI Name
Pedro Rey-Biel
PI Affiliation
ESADE
PI Name
Berkay Ozcan
PI Affiliation
NYUAD, LSE
PI Name
Sergio Schjetnan
PI Affiliation
UCL

Additional Trial Information

Status
In development
Start date
2026-06-02
End date
2026-10-31
Keywords
Behavior, Governance
Additional Keywords
JEL code(s)
Secondary IDs
Prior work
This trial is based on or builds upon one or more prior RCTs.
Abstract
We study whether ideological alignment and descriptive-norm (popularity) cues drive policy diffusion among Spanish municipalities. Municipalities receive an email offering a free Wikipedia update service and information about prior adopters. We experimentally vary (i) whether highlighted prior adopters are ideologically aligned or cross-partisan relative to the recipient, and (ii) whether the email implies high or low adoption rates. The 2×2 factorial design (plus two popularity-only arms and a pure control) allows causal identification of alignment effects, popularity-cue effects, and their interaction. The pre-analysis plan is attached as a supplementary document.
External Link(s)

Registration Citation

Citation
Capozza, Francesco et al. 2026. "Policy Diffusion, Popularity Cues, and Ideological Alignment: An RCT with Spanish Municipalities." AEA RCT Registry. May 18. https://doi.org/10.1257/rct.18667-1.0
Sponsors & Partners
Experimental Details

Interventions

Intervention(s)
Intervention Start Date
2026-06-02
Intervention End Date
2026-10-31

Primary Outcomes

Primary Outcomes (end points)
Email authorization: whether the municipality responds authorizing a Wikipedia update (click on the instruction link in the email). Baseline rate: 10.54% (García-Hombrados et al., 2024).

Wikipedia implementation: whether the municipality's Wikipedia page is actually updated following authorization. Baseline rate: 2.55%.
Primary Outcomes (explanation)

Secondary Outcomes

Secondary Outcomes (end points)
Secondary Outcomes (explanation)

Experimental Design

Experimental Design
Seven-arm experiment: a 2 (Aligned vs. Cross-partisan ideology cue) × 2 (High vs. Low popularity cue) factorial, plus two popularity-only arms (High only, Low only), and a pure control. Municipalities are assigned with equal probability (≈809 per arm). Assignment uses stratified block rerandomization (Morgan & Rubin, 2012) on four dimensions: governing-party ideology, population tertile, tourism-rate tertile, and prior treatment history. Of 1,000 candidate randomizations, the draw minimizing the maximum absolute standardized mean difference (max-ASMD < 0.10) across 13 covariates was selected (achieved max-ASMD = 0.053).
Experimental Design Details
Not available
Randomization Method
Computer
Randomization Unit
Municipalities are assigned with equal probability (≈809 per arm). Assignment uses stratified block rerandomization (Morgan & Rubin, 2012) on four dimensions: governing-party ideology, population tertile, tourism-rate tertile, and prior treatment history. Of 1,000 candidate randomizations, the draw minimizing the maximum absolute standardized mean difference (max-ASMD < 0.10) across 13 covariates was selected (achieved max-ASMD = 0.053).
Was the treatment clustered?
No

Experiment Characteristics

Sample size: planned number of clusters
na
Sample size: planned number of observations
5,666 Spanish touristic municipalities
Sample size (or number of clusters) by treatment arms
5,666 Spanish touristic municipalities with verified contact details, excluding 184 that responded in a prior round (Round 1). The universe is drawn from FEMP records and public municipal websites, restricted to municipalities meeting at least one tourism criterion (FITUR participation, coastal/border location, tourism office, registered landmark, SEGITTUR membership, or CaixaBank tourism data).
Minimum detectable effect size for main outcomes (accounting for sample design and clustering)
Alignment main effect (n ≈ 1,620/side): MDE = 3.02 pp (80% power, α = 0.05), ≈ 28% of baseline. Popularity main effect — all arms (n ≈ 2,429/side): MDE = 2.47 pp (80% power), ≈ 23% of baseline. Interaction (n ≈ 809/cell): MDE = 4.28 pp (80% power). Wikipedia implementation — alignment (n ≈ 1,620/side): ~77% power at the 1.68 pp benchmark from prior work.
Supporting Documents and Materials
IRB

Institutional Review Boards (IRBs)

IRB Name
CUHSR
IRB Approval Date
2026-05-11
IRB Approval Number
037-2025
Analysis Plan

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