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We conduct a public goods game experiment on cooperation and punishment in changing groups. We arbitrarily categorize participants into three colours: blue, red, or green*. Each participant receives an endowment of 20 monetary units and can choose to keep these units in a private account that only benefits oneself or to contribute them to two types of public goods: a colour-specific public good that gives a return of 0.7 to members of one's own colour and a global public good that gives a return of 0.5 to all members regardless of their colour. There are four members per group. In the first part of the experiment, we let participants play 10 rounds of the public goods game in their group in one of two conditions. This repeated interaction allows participants to develop group norms on how much to contribute to each of the two public goods. In one condition, groups are homogeneous in terms of colour. In the other condition, groups are heterogeneous in terms of colour (i.e., 2 members of one colour are matched with 2 members of another colour). After the first 10 rounds of the public goods game, we replace 2 members per group with 2 new members from another group and start the second part of the experiment. In the condition where groups were homogeneous prior to membership replacement, the replacement leads to heterogeneous groups (e.g., in a group with 4 blue members, 2 of the blue members are replaced by red members). We refer to this condition as HOM-HET. In the condition where groups were heterogeneous prior to membership replacement, the replacement leads to homogeneous groups (e.g., in a group with 2 blue and 2 red members, the 2 red members are replaced by 2 blue members). We refer to this condition as HET-HOM. The reshaped groups in both conditions then play another 10 rounds of the public goods game. In both conditions, we additionally vary whether peer punishment is possible, giving a total of 2 x 2 = 4 sub-conditions: 1: groups that move from homogeneity to heterogeneity due to membership change, without the option to peer-punish, 2: groups that move from heterogeneity to homogeneity due to membership change, without the option to peer-punish, 3: groups that move from homogeneity to heterogeneity due to membership change, with the option to peer-punish, 4: groups that move from heterogeneity to homogeneity due to membership change, with the option to peer-punish. * three colours are needed when there is an uneven number of groups in the laboratory and we want to switch members between groups such that each reshaped group consists of two members of one colour and two members of another colour. With an even number of groups, we only use red and blue colours.

How people cooperate to provide public goods is a major scientific question and crucial for addressing many societal problems. Previous research studied how people cooperate in stable groups in repeated or one-time-only encounters. However, most real-world public good problems occur in groups with a gradually changing composition due to old members leaving and new members arriving. How group changes are related to cooperation in public good provision is not well understood. To address this issue, we analyze a dataset from an online public goods game comprising 1.5 million contribution decisions made by 135 thousand players in 11.3 thousand groups with 234 thousand changes in group composition. We find that changes in group composition negatively relate to cooperation, mainly because newcomers contribute less than incumbents. During the process of moving from newcomer status to incumbent status, individuals cooperate more and more in line with incumbents.

We are conducting a public goods game experiment on cooperation and punishment in changing groups. Subjects will be categorized by their ethnicity (i.e., Israeli Jews and Israeli/Palestinian Arabs). Each subject receives an endowment of 20 monetary units and can choose to keep these units in a private account that only benefits oneself or to contribute them to two types of public goods: a local public good that gives a return of 0.7 to members of one's own ethnicity and a global public good that gives a return of 0.5 to all members regardless of their ethnicity. There are four members per group. In the first part of the experiment, we let subjects play 10 rounds of the public goods game in their group in one of two conditions. This repeated interaction allows subjects to develop group norms on how much to contribute to each of the two public goods. In one condition, groups begin homogeneous in terms of ethnicity. In the other condition, groups begin heterogeneous in terms of ethnicity (i.e., 2 Arabs and 2 Jews). After the first 10 rounds of the public goods game, we replace 2 members per group with 2 new members from another group and start the second part of the experiment. In the condition where groups began homogenous, they now become heterogeneous (e.g., after the first 10 rounds of the game, in groups with 4 Arab members, 2 of the Arab members are replaced by 2 Jewish members). We refer to this condition as HOM-HET. In the condition where groups began heterogeneous, the replacement leads to homogenous groups (e.g., in a group with 2 Arab and 2 Jewish members, the 2 Jewish members are replaced by 2 Arab members). We refer to this condition as HET-HOM. The reshaped groups in both conditions then play another 10 rounds of the public goods game. In both conditions, following each round of the public goods game, subjects can pay a fee to punish other group members by reducing points from their income.

Cooperation in society is often regulated by social norms prescribing how people should behave. However, with every cooperative norm there are always some individuals violating the norm. For example, although most people think it is appropriate to pay for their use of public transport and do so, there are also some individuals who free-ride. Two strands of research have examined how people react to such norm violators. The first strand examined reactions that uphold the norm, i.e., norm enforcement. For example, individuals may verbally disapprove of individuals that enter a train or bus without checking in. The second strand of research examined how people that observe norm violations may be more likely to engage in norm-violating behaviors themselves. For example, individuals may help free-riders in public transport by alerting them when conductor is nearby. So far, these two opposing types of reactions have been studied separately. We have little understanding of the factors that cause individuals to be more or less likely to respond to norm violators with behaviors that either uphold the norm or help the norm violators. In this study, we examine the role of the norm violator’s group membership. Group membership plays a role in how people interpret the intentions behind others’ behaviors, including others’ norm violations. Intergroup attribution theory suggests that people are more likely to assume good reasons for ingroup members’ norm violations and ill intentions for outgroup members’ norm violations. For example, when observing a free-rider in public transport, people may be more likely to think the norm violator forgot to pay if the violator belongs to their ingroup, whereas they may be more likely to think the violator was unwilling to pay when the violator belongs to an outgroup. These attributions can shape how people will react to the norm violator; if they assume good intentions, they are more likely to help the norm violator. If they assume bad intentions, they may be more likely to disapprove of the norm violator. Intergroup attribution biases may thus drive reactions to norm violators, with more leniency towards ingroup norm violators than outgroup norm violators.