Even if all the physical barriers to communication were known and removed, there would remain many psychological barriers to the free flow of ideas. For example, interested people acquire more information than the uninterested; people seek the sort of facts which are congenial to their existing attitudes; different groups interpret the same information differently.

If all persons provided equal targets for exposure, and the sole determinant of public knowledge were the magnitude of the given information, there would be no reason for the same individuals always to show a relative lack of knowledge. Instead, there is something about the uninformed which makes them harder to reach, no matter what the level or nature of the information.