Its subtitle: Why the clustering of Like-Minded America is Tearing Us Apart.
In a place like Colorado, the clustering has been reinforced by the immigration of lots of college-educated hipsters to the state.
Old English clyster "cluster," probably from the same root as clot (n.). Of stars, from 1727. Cluster-bomb attested from 1967.
late 14c. (transitive), from cluster (n.). Intransitive sense from 1540s. Related: Clustered; clustering.