Fig. 1: The Intergroup Attacker-Defender Contest with Exit (IADC-E).

a Participants were either assigned the role of attackers (red) or defenders (blue). In each round, participants decided how many of their 20 Experimental Money Units (EMU) to contribute to their conflict pool (the sword symbolizes the total EMU contributed by the attackers to the conflict pool, while the shield symbolizes the total EMU contributed by the defenders). If contributions to the attacker pool exceeded contributions to the defender pool, attackers won the conflict and received all non-contributed EMU from the defenders (i.e., defenders earned nothing). If defenders contributed more or equal EMU, they defended themselves successfully and everyone earned their non-contributed EMU. b In each round, defenders were simultaneously given the option to leave. If defenders left (light blue), they evaded the attack by the other group. However, leaving was costly, such that defenders who left earned 20 EMU minus the cost of leaving (L). If defenders did not leave (dark blue), they faced the attackers and decided how many of their 20 EMU to contribute to their conflict pool. Defenders always faced three attackers. Before deciding how many EMU to contribute to conflict, everyone learned how many defenders left. If no defenders left, they played the 3 vs 3 attacker-defender game shown in (a). However, if one or two defenders left, the attackers outnumbered the defenders, thereby increasing the chances of defeat for the remaining defenders. If everyone in the defender group left, there was no possibility to contribute to conflict. Consequently, everyone in the defender group earned 20 EMU (their endowment) minus the cost of leaving (L), and attackers earned 20 EMU (i.e., they simply kept their endowment).