Robert A. Heinlein Quote

Mass psychology is not simply a summation of individual psychologies; that is a prime theorem of social psychodynamics—not just my opinion; no exception has ever been found to this theorem. It is the social mass-action rule, the mob-hysteria law, known and used by military, political, and religious leaders, by advertising men and prophets and propagandists, by rabble rousers and actors and gang leaders, for generations before it was formulated in mathematical symbols. It works.

Methuselah's Children Part 1, Chapter 1 (p. 535). Originally published in Astounding Science Fiction, July - September 1941. - The Past Through Tomorrow (1967)