  Collective leadership and herding may arise in standard models of opinion
dynamics as an interplay of a strong separation of time scales within the
population and its hierarchical organization. Using the voter model as a simple
opinion formation model, we show that, in the herding phase, a group of agents
become effectively the leaders of the dynamics while the rest of the population
follow blindly their opinion. Interestingly, in some cases such herding
dynamics accelerates the time to consensus, which then become size independent
or, on the contrary, makes the consensus nearly impossible. These new behaviors
have important consequences when an external noise is added to the system that
makes consensus (absorbing) states to disappear. We analyze this new model
which shows an interesting phase diagram, with a purely diffusive phase, a
herding (or two-states) phase, and mixed phases where both behaviors are
possible.
