Why does an individual exhibit traits within a crowd that they do not display when they are alone? Why, under the influence of the mass, for example, do their mental abilities diminish, have extreme emotional reactions, or demonstrate selflessness and self-sacrifice? Freud considers this the essential question of mass psychology. He does not engage with it only because "mass phenomena" begin to dominate the society of his time, nor because in the mature period of his thought he aspires to form a broader anthropological approach, beyond the pathology of the individual patient. For him, individual psychology cannot exist separate from the influence of others; it ultimately emerged from mass psychology. Thus, by exploring the theories of his time on the formation of the mass, Freud will argue that its formation is primarily due to libidinal relationships. He will continue his daring excursions into prehistory, where he will encounter the formation of the primal mass and the primitive horde, in order to explain the emotional relationships between individuals and with the leader, starting from the murder of the primal father.
Concepts such as identification, as well as a new level of the Ego, the Ego ideal, will be used to unravel the mystery of the influence that leaders exert on individuals within the mass. The concept of hypnosis, to which Freud returns, will also be used to reaffirm its privileged but enigmatic relationships with the unconscious.