  Here we investigate the dynamics of indirect reciprocity on networks, a type
of social dynamics in which the attitude of individuals, either cooperative or
antagonistic, toward other individuals changes over time by their actions and
mutual monitoring. We observe an absorbing state phase transition as we change
the network's link or edge density. When the edge density is either small or
large enough, opinions quickly reach an absorbing state, from which opinions
never change anymore once reached. In contrast, if the edge density is in the
middle range the absorbing state is not reached and the state keeps changing
thus being active. The result shows a novel effect of social networks on
spontaneous group formation.
