(1) Citizens employ socially supplied information as a labor saving device. By finding well informed individuals with political biases similar to their own, citizens are able to reduce information costs by relying on individuals who are readily available within their own networks of social contact. Hence, the likelihood of disagreement is reduced because individuals rely on the guidance of politically compatible experts, and political homogeneity is thus the naturally occurring state of affairs within communication networks.
The problem is not that citizens engage in free riding by making use of the efforts expended by others in the collection and analysis of political information. This allows people to acquire useful information through the process of division of labor. To the extent that every citizen spends serious time and effort staying informed about politics and policy - reading newspapers, diligently watching television shows, reading Congressional bills, reading wonkish studies-- we would expect a serious erosion in the social resources available to coach little league temas, to organize Girl Scout cookie drives, to attend church council meetings, and so on. Citizenship activities do not end at the voting booth or for that matter in the world of partisan politics, elections, and public affairs.
The problem is rather that the search for a political expert to free ride on, citizens search for someone who shares their particular points of political orientation. They are never forced to consider new, novel, and perhaps uncomfortable political ideas. Rather than a community that responds to compelling arguments and changing circumstances, the community is organized into political groupings surrounded by non-permeable social and political boundaries.
(2) The psychic discomfort of disagreement causes individuals to reduce dissonance through various means. In particular, individuals adopt socially prevalent viewpoints, and they avoid disagreement in the first place by censoring their patterns of social interaction to create politically homogeneous networks of political communication.
An an experiment, an individual subject participates in a small group experiment where all the other members of the small group cooperate in a hoax. Except for this one subject, all the other individuals are instructed to provide wrong answers to an exercise that involves matching the length of various lines. The true subject must confront the fact that other group members (the bogus subjects) are giving answers contrary to the subject's own sensory perception. In this setting, the individuals often (but not always) go along with the group judgement, thereby denying their own sensory judgment.
Humans reduce dissonance in several ways. They may adapt their own beliefs to the beliefs of others. They may avoid political discussion and potential discussants whose ideas differ from theirs. They may reinterpret the statements of others to reconcile them with their own viewpoints.