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Field
Randomization Method
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Before
Participants are randomly assigned to treatments by the experimental software.
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After
Participants are randomly assigned to treatments by the experimental software.
In Sessions 1–9, treatment assignment was performed by independent random assignment (each participant independently and uniformly assigned to one of the available treatments). Starting from Session 10, we use rerandomization to improve covariate balance across treatment arms.
Specifically, we apply a simple multi-arm extension of the Euclidean rerandomization scheme proposed by Schindl and Branson (2025) and recommended by List (2026, p. 191), using participants’ prosocial choice rates (share of Action A) from Game Rounds 1–3 as balancing covariates. Treatment assignments are drawn until a Euclidean balance criterion on standardized covariates falls below a threshold
calibrated to an acceptance probability of 0.1%.
The new algorithm also attempts to place participants into treatments in approximately equal numbers. Should the Euclidean balancing algorithm fail for any reason, the code automatically falls back to a Completely Randomized Experiment (List, 2026, section 6.3.2).
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