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Students responded to the opportunity to take part in a study for course credit plus a monetary bonus. Study sessions were scheduled in groups of at least 12 and up to 24. In total, we ran 20 networks with 334 unique participants. Network sizes ranged from 12 to 24 (M = 16.7), depending on the number of participants that attended the study session. This minimum, range, average, and variation are standard in past work on cooperation in networks (12, 1517).

As participants arrived at the laboratory, they were seated at an isolated computer station. Before beginning the study, participants completed a consent form detailing the study procedures and their expected payment for participating. Thereafter, a custom Web application displayed instructions for an iterated PDG. Participants began with an endowment of 1000 monetary units (MUs). Following past work (2, 12, 15, 18), cooperation entailed paying 50 MUs and resulted in the alter gaining 100 MUs. Defection entailed paying nothing and generating no benefits. Several comprehension check questions, with feedback, were included in the study instructions (see the Supplementary Materials for additional details, including screenshots of the study instructions).

The study consisted of three phases (described in more detail below). At the beginning of each phase, participants were randomly assigned to a position within a network and given a unique letter ID that was displayed throughout the phase. Initial networks were random (Erdös-Rényi) graphs with a density of 0.21, meaning that participants had 4.36 ties, on average, at the beginning of the study. In each round of the phase, participants made decisions to cooperate or defect independently for each alter to whom they were tied. Once the phase was complete, participants were randomly assigned to a position in a new random network, received a new identifier, and were given a new 1000-MU endowment before the next phase began.

We manipulated the type of network within-subjects such that each of the three conditions (or phases) was presented in random order. Following past work in studies of networks and cooperation (13, 15), in the static network condition, participants were tied to the same alters through the entire phase. In the dynamic network condition, participants could sever a tie to one alter and initiate a new tie after every three rounds of the phase. Prospective new alters included any alter not currently tied to the participant, including those the participant had dropped in previous rounds. When initiating a tie to a new alter, the alter could either approve or decline the request; only if the alter accepted did the new tie form. Ties were not replaced for participants who were dropped (2, 3, 18, 19). Any participant who lost all of their ties became an isolate and was excluded from the network for the remainder of the phase.

In our mixed network condition, we randomly assigned whether each of the participants’ individual ties was static or dynamic. Participants were aware which of their ties were static and which were dynamic when making PDG decisions. As in the dynamic network condition, after every three rounds, participants could drop one of their alters and initiate a new tie, provided the relation was dynamic. Static ties were maintained throughout the duration of the phase. New ties formed during the tie selection process were always dynamic.

In addition to manipulating the type of network, we also manipulated the presence of reputation information by either displaying reputations [i.e., the percentage of times participants had cooperated in previous rounds of the current phase (2, 4)] or not when participants selected new alters in the dynamic and mixed network conditions. As a result, participants did not see reputations in the static network condition, where participants were unable to select new ties, nor did they see reputations if they chose not to drop a tie. This manipulation was between-subjects, with 10 networks in each condition. This number of networks is typical in the literature on cooperation and networks, especially when the network size is large (2, 3, 17).

Each phase lasted 12 rounds; thus, participants completed 36 rounds in total. To avoid end-game effects, participants were not told how many rounds to expect, nor were participants in the dynamic and mixed network conditions told exactly when tie-dropping opportunities would occur, only that they would happen “periodically.” Each session lasted approximately 75 min. The Institutional Review Boards at both universities reviewed and approved the procedures.

There was no deception in the study, and we included extensive comprehension check items with feedback to ensure that participants understood the instructions. As a result, we did not expect to exclude any observations from analyses unless a computer error occurred. All analyses were conducted using the full dataset. See the Supplementary Materials for additional details.

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