Testing isomorphic invariance across social dilemma games

Can purely behavioral aspects affect voluntary cooperativeness in isomorphic social dilemma games? We experimentally test isomorphic invariance by comparing frames whose identical payoffs are described as of the Prisoners' Dilemma or the linear Public Good. Participants play two consecutive rounds of the same frame, with no between-round feedback information, interacting with either the same or a different number of other subjects in each round. Hence, frames are compared between subjects whereas games with different numerosity are compared both within subjects and between subjects. Our analysis rejects isomorphic invariance and shows significantly lower average cooperativeness in the linear Public Good frame across all conditions. Moreover, we find a significantly negative effect of group size, especially in Prisoners' Dilemma.