His work on crowd psychology became important in the first half of the twentieth century when it was used by media researchers such as Hadley Cantril and Herbert Blumer to describe the reactions of subordinate groups to media.
He also contributed to on-going debates in physics about the nature of matter and energy. His book The Evolution of Matter was very popular in France (going through twelve editions), and though some of its ideas — notably that all matter was inherently unstable and was constantly and slowly transforming into luminiferous ether — were taken up favorably by physicists of the day (including Henri Poincaré), his specific formulations were not given much consideration. In 1896 he reported observing a new kind of radiation, which he termed "black light" (not the same thing as what modern people call black light today), though it was later discovered not to exist.
Le Bon enjoyed considerable security at the center of French intellectual life thereafter. In 1902, he launched a series of weekly luncheons (les déjeuners du mercredi) to which prominent figures from all the professions were invited to discuss topical issues. The strength of Le Bon's personal networks is apparent from the guest list: participants included Henri and Raymond Poincaré (cousins, physicist and President of France respectively), Paul Valéry and Henri Bergson.
Wilfred Trotter, a famous surgeon at University College Hospital, London, wrote along similar lines in his famous book Instincts of the Herd in Peace and War, just before the outbreak of World War II; he has been referred to as 'Le Bon's popularizer in English.' Trotter also read Freud, and it was he who introduced Wilfred Bion, who worked for him at the hospital, to Freud's writings, and ultimately both he and Ernest Jones ascribed to the field of what would later be called group psychology. Both of these men became closely associated with Freud when he fled Austria shortly after the Anschluss. Both men were closely linked to the Tavistock Institute as key figures in the field of group dynamics.
It is arguable that the fascist theories of leadership that emerged in the 1920s owed much to Le Bon's theories of crowd psychology. Indeed, Hitler's Mein Kampf drew largely on the propaganda techniques proposed in Le Bon's 1895 book. Edward Bernays, a nephew of Sigmund Freud, was influenced by Le Bon and Trotter. In his famous book Propaganda he declared that a major feature of democracy was the manipulation of the mass mind by media and advertising.