Complete strategy spaces unlock hidden pathways to cooperation

A new paper from researchers including Zhao Song, Matjaz Perc, and The Anh Han, posted on arXiv (2511.17794), tackles a core question in evolutionary game theory: how cooperation emerges and persists. Previous models relied on restricted, hand-picked strategy sets—for example, a recent four-strategy framework showing cheap talk can promote cooperation through local interactions. The authors suspected that arbitrarily excluding strategies might distort the conclusions. To test this, they extended the framework to encompass the complete set of eight strategies that naturally arise from communication and decision-making rules in the 'cheap talk' game.

The results, detailed in the preprint, show that modeling the full strategy space dramatically changes the evolutionary landscape. Cooperation becomes both more robust and more versatile, thanks to novel pathways that simply do not exist in the restricted model. In particular, the team discovered a previously hidden mechanism where suspicious cooperation—a strategy that expects but also tries to exploit deceptive communication—catalyzes a cyclic dynamic that sustains cooperation across populations. Conversely, the assumed pivotal role of strategic defection turns out to be fragile: it acts mainly as a spoiler rather than a genuine evolutionary attractor. The complete model also reveals a rich spectrum of long-term behaviors, including stable coexistence among up to seven strategies and time-varying patterns of partial coexistence. These findings emphasize that comprehensive modeling is essential when explaining cooperation, and hint at new design principles for multi-agent AI systems and social institutions.