The article describes the content of the concepts of the mob, developed by representatives of American psychological thought of the late XIX - early XX centuries. A criticism of C. Cooley of previous theories created on the other side of the Atlantic is presented. The dependence of the initial views of B. Sidis on the material of the theory of "heroes and crowds" N.K. Mikhailovsky. His original ideas are analyzed about the role of suggestion in public life, the splitting of consciousness as the physiological basis of suggestibility and the conditions for the general “I” of a crowd (mob) to dominate the personal “I” of its constituent individuals. Characterized by the concept of E.A. Ross “mob mind” as a mental state. His thoughts on social “insanities” and “quirks” (craze and fad) are given. The contribution of B. Sidis and E.A. Ross in the development of scientific ideas about collective behavior.