Group size effect on cooperation in one-shot social dilemmas II. Curvilinear effect

TL;DR

This study investigates how group size influences cooperation in public goods games, revealing a curvilinear relationship where intermediate groups cooperate more than smaller or larger ones, bridging lab and field findings.

Contribution

It introduces a novel public goods game model with decelerating benefits and provides experimental evidence of a curvilinear group size effect on cooperation.

Findings

Intermediate group sizes lead to higher cooperation levels.

The effect of group size on cooperation is curvilinear.

Results bridge gap between lab and field experiments.

Abstract

In a world in which many pressing global issues require large scale cooperation, understanding the group size effect on cooperative behavior is a topic of central importance. Yet, the nature of this effect remains largely unknown, with lab experiments insisting that it is either positive or negative or null, and field experiments suggesting that it is instead curvilinear. Here we shed light on this apparent contradiction by considering a novel class of public goods games inspired to the realistic scenario in which the natural output limits of the public good imply that the benefit of cooperation increases fast for early contributions and then decelerates. We report on a large lab experiment providing evidence that, in this case, group size has a curvilinear effect on cooperation, according to which intermediate-size groups cooperate more than smaller groups and more than larger groups. In doing so, our findings help fill the gap between lab experiments and field experiments and suggest concrete ways to promote large scale cooperation among people.