  While enforcing egalitarian social norms is critical for human society,
punishing social norm violators often incurs a cost to the self. This cost
looms even larger when one can benefit from an unequal distribution of
resources (i.e. advantageous inequity), as in receiving a higher salary than a
colleague with the identical role. In the Ultimatum Game, a classic test bed
for fairness norm enforcement, individuals rarely reject (punish) such unequal
proposed divisions of resources because doing so entails a sacrifice of one's
own benefit. Recent work has demonstrated that observing another's punitive
responses to unfairness can efficiently alter the punitive preferences of an
observer. It remains an open question, however, whether such contagion is
powerful enough to impart advantageous inequity aversion to individuals. Using
a variant of the Ultimatum Game in which participants are tasked with
responding to fairness violations on behalf of another 'Teacher' - whose
aversion to advantageous (versus disadvantageous) inequity was systematically
manipulated-we probe whether individuals subsequently increase their punishment
unfair after experiencing fairness violations on their own behalf. In two
experiments, we found individuals can acquire aversion to advantageous inequity
'vicariously' through observing (and implementing) the Teacher's preferences.
Computationally, these learning effects were best characterized by a model
which learns the latent structure of the Teacher's preferences, rather than a
simple Reinforcement Learning account. In summary, our study is the first to
demonstrate that people can swiftly and readily acquire another's preferences
for advantageous inequity, suggesting in turn that behavioral contagion may be
one promising mechanism through which social norm enforcement - which people
rarely implement in the case of advantageous inequality - can be enhanced.
