  Many mechanisms for the emergence and maintenance of altruistic behavior in
social dilemma situations have been proposed. Indirect reciprocity is one such
mechanism, where other-regarding actions of a player are eventually rewarded by
other players with whom the original player has not interacted. The upstream
reciprocity (also called generalized indirect reciprocity) is a type of
indirect reciprocity and represents the concept that those helped by somebody
will help other unspecified players. In spite of the evidence for the
enhancement of helping behavior by upstream reciprocity in rats and humans,
theoretical support for this mechanism is not strong. In the present study, we
numerically investigate upstream reciprocity in heterogeneous contact networks,
in which the players generally have different number of neighbors. We show that
heterogeneous networks considerably enhance cooperation in a game of upstream
reciprocity. In heterogeneous networks, the most generous strategy, by which a
player helps a neighbor on being helped and in addition initiates helping
behavior, first occupies hubs in a network and then disseminates to other
players. The scenario to achieve enhanced altruism resembles that seen in the
case of the Prisoner's Dilemma game in heterogeneous networks.
