# The key role of cheaters in the persistence of cooperation

## TL;DR

This paper shows that cheaters can help, rather than hinder, the evolution of cooperation in populations through multi-level selection.

## Contribution

The study reveals that cheaters can promote cooperation by enabling population growth at the group level.

## Key findings

- Cheaters provide a reproductive advantage at the individual level but can act altruistically at the group level.
- The presence of cheaters positively correlates with the survival of cooperators under relative fitness advantage.
- Agent-based models confirm that cheaters facilitate the evolution of new spatial organization in bacterial biofilms.

## Abstract

Evolution of cooperation is a major, extensively studied problem in evolutionary biology. Cooperation is beneficial for a population as a whole but costly for the bearers of social traits such that cheaters enjoy a selective advantage over cooperators. Here, we focus on coevolution of cooperators and cheaters in a multi-level selection framework, by modeling competition among groups composed of cooperators and cheaters. Cheaters enjoy a reproductive advantage over cooperators at the individual level, independent of the presence of cooperators in the group. Cooperators carry a social trait that provides a fitness advantage to the respective groups.

In the case of absolute fitness advantage, where the survival probability of a group is independent of the composition of other groups, the survival of cooperators does not correlate with the presence of cheaters. By contrast, in the case of relative fitness advantage, where the survival probability of a group depends on the composition of all groups, the survival of cooperators positively correlates with the presence of cheaters. Increasing the strength of the social trait alone fails to ensure survival of cooperators, and the increase of the reproduction advantage of the cheaters is necessary to avoid population extinction. This unexpected effect comes from multilevel selection whereby cheaters at the individual level become altruists at the group level, enabling overall growth of the population that is essential for the persistence of cooperators. We validate these theoretical results with an agent-based model of a bacterial biofilm where emergence of the cooperative trait is facilitated by the presence of cheaters, leading to evolution of new spatial organization.

Our results suggest that, counterintuitively, cheaters often promote, not destabilize, evolution of cooperation.

The online version contains supplementary material available at 10.1186/s12915-025-02492-5.