Le Pen's most famous book Mob: A Study of Popular Psychology was published in 1895. He believes that the behavior of groups is essentially different from that of individuals. There is a kind of ideological unity in gathering, which Le Pen calls "the law of psychological unity of the crowd", which can be manifested as unbearable, irresistible force or irresponsibility. Group behavior may be sudden and extreme; The intellectual process may be preliminary and mechanical. This is one of several popular theories of "group psychology" at that time. In the case of clustering, individuals give up their independent critical thinking ability, let the spirit of the group replace their own spirit, and then give up their sense of responsibility and even various constraints. The most rational people will act like animals. There will also be ideological infection when clustering, which will make the unconscious thoughts of partisans and the masses penetrate each other through a mysterious thing. Le Pen concluded that when it becomes a collective, any weak personal belief may be strengthened.

Le Pen's thought has a great influence on analytical social psychology, and at the same time it has become an indispensable content in the study of modern ideology.