  The constrained voter model describes the dynamics of opinions in a
population of individuals located on a connected graph. Each agent is
characterized by her opinion, where the set of opinions is represented by a
finite sequence of consecutive integers, and each pair of neighbors, as defined
by the edge set of the graph, interact at a constant rate. The dynamics depends
on two parameters: the number of opinions denoted by $F$ and a so-called
confidence threshold denoted by $\theta$. If the opinion distance between two
interacting agents exceeds the confidence threshold then nothing happens,
otherwise one of the two agents mimics the other one just as in the classical
voter model. Our main result shows that the one-dimensional system starting
from any product measures with a positive density of each opinion fluctuates
and clusters if and only if $F \leq 2 \theta + 1$. Sufficient conditions for
fixation in one dimension when the initial distribution is uniform and lower
bounds for the probability of consensus for the process on finite connected
graphs are also proved.
