Werner Güth, Matteo Ploner and Tobias Regner

In--group favoritism in social dilemma situations is one of the main
findings of studies in Social Identity Theory. We investigate what
causes the in--group bias: is it due to mere group affiliation or,
alternatively, is guilt--aversion a possible explanation? We induce
group membership in a minimal group setting, observe
in--/out--group transfers and elicit respective beliefs. We find
that mere group affiliation explains a substantial part of the
bias, but we also find evidence in favor of guilt--aversion as a source of
motivation.