Inequality, outreach and impact in public goods contributions

Last registered on November 11, 2021


Trial Information

General Information

Inequality, outreach and impact in public goods contributions
Initial registration date
May 31, 2021

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 01, 2021, 10:28 AM EDT

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

Last updated
November 11, 2021, 12:15 AM EST

Last updated is the most recent time when changes to the trial's registration were published.



Primary Investigator

University of Pretoria

Other Primary Investigator(s)

Additional Trial Information

Start date
End date
Secondary IDs
Prior work
This trial does not extend or rely on any prior RCTs.
We use economics experimental games to investigate aspects of decision making pertaining to giving. Specifically, we investigate how willingness to give is affected by inequality, outreach and the impact of donations.
External Link(s)

Registration Citation

Nicholls, Nicky. 2021. "Inequality, outreach and impact in public goods contributions." AEA RCT Registry. November 11.
Experimental Details


Participants take part in 4 decision making tasks. For one of the tasks, participants are either assigned to a high or low endowment group. For the other 3 tasks, all participants receive the same endowment, but the specifics of the tasks vary.
Intervention Start Date
Intervention End Date

Primary Outcomes

Primary Outcomes (end points)
Willingness to contribute in different experimental scenarios
Primary Outcomes (explanation)
Willingness to contribute will be measured as chosen contributions in incentivised public goods games and incentivised dictator games with a charity as the receiver.

Secondary Outcomes

Secondary Outcomes (end points)
Secondary Outcomes (explanation)

Experimental Design

Experimental Design
An online survey will be conducted, where respondents make decisions in 2 public goods games and 2 dictator games. Dimensions of interest are varied across these games. The decisions are incentivised.
Experimental Design Details
To allow us to test our hypotheses, respondents will first be asked to participate in 4 experimental tasks to investigate their contributions to public goods in different scenarios. The order of these decisions is randomised to reduce any order effects, and the order of presentation will also be controlled for in the regression analysis. The tasks include two public goods game tasks, where these tasks follow the typical public goods game set-up: respondents are given a monetary endowment, and are asked to stipulate an amount (which can be zero) from this endowment that they would like to put into a group investment. Participants are informed that contributions from the four members of their group (including themselves) are doubled, and that the resulting amount will be divided equally among the four group members. Any amount that participants do not contribute to the group investment are kept. Since we use a multiplier of 2, this gives a marginal per capita return on investment of 0.5. That is, our public goods game follows the common approach where the dominant strategy equilibrium outcome for participants concerned only with their own payment would be to contribute nothing; while the social optimum is for all participants to contribute their full endowment. One of the public goods game tasks has all four participants receive the same endowment: R50. The other has unequal endowments, where half of the participants are given an endowment of R30 and the other half of the respondents are given an endowment of R60. In this task, all participants know that their group includes 2 participants with R30 and 2 participants with R60.

The other two tasks use a modified dictator game, where participants again receive an endowment, and must now choose how much of the endowment to keep and how much to contribute to a public good, in this case the Solidarity Fund. We vary the impact of contributions in these tasks by having two scenarios. The first allows for a direct comparison with the equal endowment public goods game: participants receive an endowment of R50, and any contributions are multiplied by 2. The second scenario allows us to directly compare contributions with higher and lower impacts: here the endowment remains R50, but contributions are muliplied by 4, doubling the impact of each rand contributed.

In all cases, examples are used to explain the payments resulting from possible decisions. To reduce possible experimenter demand effects, examples include a range of possible contributions.

To allow us to investigate the impact of beliefs about others' contributions on contribution decisions, we ask participants to report what they believe to be the average contribution of other participants for each of the games. This task is incentivised by paying the participant guessing closest to the true average for each game an incentive of R50.

In addition to these experiment questions, we ask participants to complete a short survey on their household income, education level and demographics (age, gender, race). We also ask respondents whether they own a small business, and how the small business has fared following Covid-19 and the associated lockdowns. This will allow us to see whether people who stand to benefit directly from the Solidarity Fund are more or less likely to be willing to contribute to this fund.
Randomization Method
Data will be collected in qualtrics, an online survey software. This software allows for respondents to be randomly assigned to either the high or low endowment group.
Randomization Unit
Was the treatment clustered?

Experiment Characteristics

Sample size: planned number of clusters
900 individuals
Sample size: planned number of observations
900 individuals
Sample size (or number of clusters) by treatment arms
450 individuals in high endowment, 450 individuals in low endowment
Minimum detectable effect size for main outcomes (accounting for sample design and clustering)

Institutional Review Boards (IRBs)

IRB Name
University of Pretoria Economic and Management Sciences Ethics Committee
IRB Approval Date
IRB Approval Number
EMS 205/20
Analysis Plan

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Post Trial Information

Study Withdrawal

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Is the intervention completed?
Intervention Completion Date
September 06, 2021, 12:00 +00:00
Data Collection Complete
Data Collection Completion Date
September 06, 2021, 12:00 +00:00
Final Sample Size: Number of Clusters (Unit of Randomization)
900 units (1615 started survey, respondents' surveys were terminated if they did not pass the understanding/attention check question or if they were under 18 or did not reside in South Africa)
Was attrition correlated with treatment status?
Final Sample Size: Total Number of Observations
900 individuals
Final Sample Size (or Number of Clusters) by Treatment Arms
900 individuals, 453 high treatment, 447 low treatment (in inequality question)
Data Publication

Data Publication

Is public data available?

Program Files

Program Files
Reports, Papers & Other Materials

Relevant Paper(s)

Reports & Other Materials