Homogeneity of opinion among groups of people is mostly caused by

Homogeneity of opinion among groups of people is mostly caused by inter-personal communication.

Face-to-face exchange within a group lets members influence one another until their views converge.

Mass communication reaches scattered, heterogeneous audiences and does not by itself unify opinion.

Intra-personal communication happens within a single individual and cannot shape a whole group.

Abstraction in communication concerns levels of generality, not the forming of shared group opinion.

Discussion and mutual influence among peers is what draws opinions into agreement.