  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.
