Seen One, Seen Them All

Last registered on June 22, 2026
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Pre-Trial

Trial Information

General Information

Title
Seen One, Seen Them All
RCT ID
AEARCTR-0018621
Initial registration date
June 15, 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
June 22, 2026, 6:44 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
Mathilde Lea Editha Bechdolf
Affiliation
Otto-von-Guericke-Universität (OVGU) Magdeburg
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Other Primary Investigator(s)

PI Name
Abdolkarim Sadrieh
PI Affiliation
Otto von Guericke University Magdeburg
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PI Name
Max Grossmann
PI Affiliation
University of Melbourne
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Additional Trial Information

Status
In development
Start date
2026-06-15
End date
2027-06-15
Keywords
Behavior, Lab
Additional Keywords
Intergroup Bias, Biased Beliefs, Social Identity
JEL code(s)
C72, C92, D83, D91
Secondary IDs
Prior work
This trial does not extend or rely on any prior RCTs.
Abstract
When interacting with others, individuals are uncertain about whether they will encounter pro-social or anti-social behavior. In such interactions, even a single observation can trigger biased beliefs about entire groups. We conduct a laboratory experiment in which participants simultaneously receive statistically irrelevant information about their in-group and out-group, with treatments varying whether both signals are pro-social or anti-social. Since the informational structure is identical across groups by design, any subsequent difference in cooperative behavior toward in-group versus out-group members must reflect asymmetric biased beliefs. We hypothesize that individuals overweigh signals that confirm existing group stereotypes, generalizing negative information about out-group members to the out-group as a whole, while dismissing equivalent negative information about in-group members as an isolated case. Group identities are induced via the minimal group paradigm. Participants play three cooperation games, a Prisoner's Dilemma, a Stag Hunt, and a Mini Trust Game, separately with in-group and out-group members before and after receiving the anecdotal information. Understanding how statistically irrelevant information generates systematic discrimination is relevant for policy makers seeking to counteract stereotype formation, intergroup hostility, and political polarization.
External Link(s)

Registration Citation

Citation
Bechdolf, Mathilde Lea Editha, Max Grossmann and Abdolkarim Sadrieh. 2026. "Seen One, Seen Them All." AEA RCT Registry. June 22. https://doi.org/10.1257/rct.18621-1.0
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Experimental Details

Interventions

Intervention(s)
Participants take part in four Game Rounds in which they take several decisions in three 2-player games: a Prisoner's Dilemma, a Stag Hunt, and a Mini Trust Game. After Game Round 1, group identities are induced using the minimal group paradigm: participants are assigned to either the Klee group or the Kandinsky group based on their painting preferences. From Game Round 3 onwards, participants know whether their decisions are matched with a member of their own group (in-group) or the other group (out-group).
Participants are randomly assigned to one of three treatments. In the Good and Bad treatments, participants receive anecdotal information about one decision made by one Klee group member and one Kandinsky group member in Game Round 2. The information indicates either cooperative behavior (Good) or uncooperative behavior (Bad) in the Prisoner's dilemma. The Baseline group receives no information.
Intervention Start Date
2026-06-15
Intervention End Date
2027-06-15

Primary Outcomes

Primary Outcomes (end points)
The in-group cooperation advantage and mean cooperation rate, both measured in Game Rounds 3 and 4.
Primary Outcomes (explanation)
For each participant, game, and game round, a cooperation rate is computed as the share of prosocial choices out of all decisions made. In Game Rounds 3 and 4, where group identity is visible, cooperation rates are computed separately for decisions matched with in-group member decisions and decisions matched with out-group member decisions.
The in-group cooperation advantage is defined as the difference between the in-group cooperation rate and the out-group cooperation rate. Mean cooperation is defined as the average of the two. Both outcomes are averaged across the three games to obtain pooled measures, and also analyzed separately for each game.

Secondary Outcomes

Secondary Outcomes (end points)
Cooperation rate in Game Round 1 and Game Round 2. General preferences (risk willingness, reciprocity, trust), attitudes on social and economic topics, fairness and inequality preferences, personality traits (Mini-IPIP Big Five), and demographic characteristics (age, gender, field of study, country of higher education entrance qualification).
Secondary Outcomes (explanation)
For each participant and game, a cooperation rate is computed as the share of prosocial choices out of all decisions made in Game Round 1 and Game Round 2, where group identity is not yet visible. These outcomes are used to assess whether the introduction of group identity alone affects cooperation, by comparing Game Round 1 and Game Round 2.
General preferences, attitudes, fairness and inequality preferences, and personality traits are elicited in a pre-survey completed online before the laboratory session. Demographic characteristics are collected after the experiment.

Experimental Design

Experimental Design
Participants take part in four Game Rounds in which they take several decisions in three 2-player games: a Prisoner's Dilemma, a Stag Hunt, and a Mini Trust Game. In each game, participants choose between Action A and Action B; Action B is the non-prosocial option in all three games. The three games are played in random order within each Game Round. In each Game Round, participants make 20 decisions per game, resulting in 60 decisions per Game Round. Before each Game Round, participants' expectations about the share of prosocial decisions are elicited.
In Game Round 1, participants take 60 decisions matched with decisions of other participants in the session of Game Round 1.
After Game Round 1, social identity is induced using the minimal group paradigm.
In Game Round 2, participants know their own social identity but opponent identity is not shown. Participants take 60 decisions matched with decisions of other participants in the session of Game Round 2.
In Game Round 3, opponent identity is revealed. For each game, participants make 10 decisions matched with in-group member decisions and 10 decisions matched with out-group member decisions of Game Round 2.
Between Game Round 3 and Game Round 4, participants are randomly assigned to one of three treatments, subject to a no-deception constraint: the Good and Bad treatments are only available when the corresponding statements are true based on Game Round 2 data.
Baseline: no information shown
Good: participants are informed that one Klee group member and one Kandinsky group member chose the prosocial option in the Prisoner's Dilemma in Game Round 2
Bad: participants are informed that one Klee group member and one Kandinsky group member chose the non-prosocial option in the Prisoner's Dilemma in Game Round 2
In Game Round 4, participants again make 10 decisions matched with in-group member decisions and 10 decisions matched with out-group member decisions of Game Round 2.
Experimental Design Details
Not available
Randomization Method
Participants are randomly assigned to treatments by the experimental software.
Randomization Unit
Individual participant.
Was the treatment clustered?
No

Experiment Characteristics

Sample size: planned number of clusters
750 participants
Sample size: planned number of observations
750 participants
Sample size (or number of clusters) by treatment arms
250 participants Baseline, 250 participants Good, 250 participants Bad
Minimum detectable effect size for main outcomes (accounting for sample design and clustering)
Supporting Documents and Materials
IRB

Institutional Review Boards (IRBs)

IRB Name
Institutional Review Board Certificate of the German Association for Experimental Economic Research e.V,
IRB Approval Date
2026-05-13
IRB Approval Number
XRrnNipD
IRB Name
Human Research Safeguards, Office of Research Ethics and Integrity, The University of Melbourne
IRB Approval Date
2026-05-14
IRB Approval Number
34596 (2026-34596-80344-6)
Analysis Plan

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