Cooperation in repeated Prisoner's dilemma experiments

Last registered on May 17, 2023


Trial Information

General Information

Cooperation in repeated Prisoner's dilemma experiments
Initial registration date
May 10, 2023

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 17, 2023, 2:16 PM EDT

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


Primary Investigator

University of Nottingham

Other Primary Investigator(s)

PI Affiliation
University of Nottingham
PI Affiliation
University of Nottingham
PI Affiliation
University of Nottingham

Additional Trial Information

In development
Start date
End date
Secondary IDs
Prior work
This trial does not extend or rely on any prior RCTs.
We study behavior in finitely repeated prisoner's dilemma experiments, where fixed pairs of players will interact for 100 rounds. The research question is how payoff parameters affect cooperation. Specifically, we vary the efficiency of cooperation and the temptation to defect in an orthogonal 2x2 design. The null hypothesis is that cooperation does not differ between treatments. The alternative hypothesis is that cooperation should be higher and more stable in "easy" games, where efficiency is high and temptation to defect is low, than in "hard" games, where efficiency is low and temptation is high. We expect cooperation in the other two treatments (high efficiency, high temptation; and low efficiency, low temptation) to be between the hard and easy games. We plan to recruit 40 pairs for each treatment, that is, a total of 320 volunteer participants registered with the CeDEx lab at the University of Nottingham. The experiment will be computerized with the experimental software zTree and conducted in the CeDEx lab.
External Link(s)

Registration Citation

Fang, Tong et al. 2023. "Cooperation in repeated Prisoner's dilemma experiments." AEA RCT Registry. May 17.
Experimental Details


Participants will play 100 rounds of a two-person symmetric Prisoner's dilemma with a randomly allocated fixed partner. The prisoner's dilemma is characterised by four parameters: R(eward), T(emptation), P(unishment), S(ucker), where the following order holds: T > R > P > S and 2R > T + S. in each round, participants will simultaneously choose either C(ooperate) or D(efect), which results in four possible outcomes:
1) C, C, which results in payoffs (R, R)
2) D, C, which results in payoffs (T, S)
3) C, D, which results in payoffs (S, T)
4) D, D, which results in payoffs (P, P)

Our intervention is to run four treatments by orthogonally varying parameters (T, R, P, S) denoted in Experimental Currency Units (ECU) and by allocating pairs of participants at random to treatment. The treatments are described in the Experimental Design section below.

Our research question is how the payoff structure that we vary orthogonally affects cooperation rates.
Intervention Start Date
Intervention End Date

Primary Outcomes

Primary Outcomes (end points)
The primary outcomes is the cooperation rates we observe as a function of treatment in the experiment (the fraction of times players choose C.
Primary Outcomes (explanation)

Secondary Outcomes

Secondary Outcomes (end points)
Secondary Outcomes (explanation)

Experimental Design

Experimental Design
We will run four treatments which orthogonally vary the prisoner's dilemma parameters (T, R, P, S) denoted in Experimental Currency Units (ECU) (see also the section on Interventions):

1) (600, 500, 200, 50)
2) (800, 500, 400, 100)
3) (800, 500, 200, 50)
4) (600, 500, 400, 100)

Participants will be randomly allocated to a partner and play the 100 rounds with the same partner. Pairs are randomly allocated to one of the four treatments, that is, we use a between-subjects design. Participants will play only one set of 100 rounds, after which the experiment is finished and followed by a short demographic exit questionnaire.

Participants will be paid according to all their decisions during the experiment. Accumulated ECUs are exchanged into British Pounds at an exchange rate of £0.01 per 25 ECU.

The experiments will take place in the dedicated CeDEx research labs at the University of Nottingham. Participants will be recruited from a database of registered volunteer participants. Participation is voluntary and subject to informed consent.

Experimental Design Details
Randomization Method
Upon entering the lab, participants will be randomly allocated to a numbered work station in the computer laboratory by drawing a card that indicates the number of the work station for a participant. During the experiment, pairs of players are randomly matched by the software, which also randomly allocates pairs to one of the four treatment conditions.
Randomization Unit
For randomly allocating participants to work stations in the lab the unit is the individual. For random allocation to treatment the unit is the randomly matched pair.

Was the treatment clustered?

Experiment Characteristics

Sample size: planned number of clusters
Our planned sample size is 40 pairs for each of the four treatments, that is, a total of 160 pairs (=clusters).
Sample size: planned number of observations
We plan on 320 individuals (160 pairs), that is 80 per treatment, resulting in 40 pairs per treatment
Sample size (or number of clusters) by treatment arms
40 pairs per treatment
Minimum detectable effect size for main outcomes (accounting for sample design and clustering)

Institutional Review Boards (IRBs)

IRB Name
Nottingham School of Economics Research Ethics Committee
IRB Approval Date
IRB Approval Number


Post Trial Information

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Is the intervention completed?
Data Collection Complete
Data Publication

Data Publication

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Program Files

Program Files
Reports, Papers & Other Materials

Relevant Paper(s)

Reports & Other Materials