Abstract
Humans everywhere cooperate in groups to achieve benefits not attainable by individuals. Individual effort is often not automatically tied to a proportionate share of group benefits. This decoupling allows for free-riding, a strategy that (absent countermeasures) outcompetes cooperation. Empirically and formally, punishment potentially solves the evolutionary puzzle of group cooperation. Nevertheless, standard analyses appear to show that punishment alone is insufficient, because second-order free riders (those who cooperate but do not punish) can be shown to outcompete punishers. Consequently, many have concluded that other processes, such as cultural or genetic group selection, are required. Here, we present a series of agent-based simulations that show that group cooperation sustained by punishment easily evolves by individual selection when you introduce into standard models more biologically plausible assumptions about the social ecology and psychology of ancestral humans. We relax three unrealistic assumptions of past models. First, past models assume all punishers must punish every act of free riding in their group. We instead allow punishment to be probabilistic, meaning punishers can evolve to only punish some free riders some of the time. This drastically lowers the cost of punishment as group size increases. Second, most models unrealistically do not allow punishment to recruit labor; punishment merely reduces the punished agent’s fitness. We instead realistically allow punished free riders to cooperate in the future to avoid punishment. Third, past models usually restrict agents to interact in a single group their entire lives. We instead introduce realistic social ecologies in which agents participate in multiple, partially overlapping groups. Because of this, punitive tendencies are more expressed and therefore more exposed to natural selection. These three moves toward greater model realism reveal that punishment and cooperation easily evolve by direct selection—even in sizeable groups.
Highlights
IntroductionPeople band together to defend their village from raids
As we show in a simple model, once these real world considerations are taken into account, the free rider problem can be solved—and group cooperation can evolve—without cultural group selection
Even rare punishment can induce much cooperation. The results of these simulations are clear: Under minimal yet realistic assumptions, the motivation to punish defections in group cooperation can readily evolve and can stabilize the evolution of motivations to cooperate by default—that is, without being induced to do so by the imminent threat of being punished
Summary
People band together to defend their village from raids. They build communal irrigation systems to water their crops. They collaborate in academic teams to publish scholarly papers. Our hunter-gatherer ancestors typically shared food widely to cover one another’s foraging shortfalls—a pattern believed to be characteristic of our lineage for hundreds of thousands if not millions of years [1]. In every known society past and present, people in groups made and make sacrifices to produce shared benefits. Humans achieve high-value outcomes that otherwise would have remained out of reach
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