He was interested in the study of a wide variety of sexual and erotic urges, at a time when the early taxonomy of sexual identity labels was still being formed. His scientific work extended that of Karl Heinrich Ulrichs and influenced Havelock Ellis and Edward Carpenter. He often visited bars in Berlin catering to gays and transvestites as he researched the first-ever book on transgenderism, Die Transvestiten (1910).
In 1921 Hirshfeld organised the First Congress for Sexual Reform, which led to the formation of the World League for Sexual Reform. Congresses were held in Copenhagen (1928), London (1929), Vienna (1930), and Brno (1932).
Hirschfeld was both quoted and caricatured in the press as a vociferous expert on sexual manners, receiving the epithet “the Einstein of Sex”. He saw himself as a campaigner and a scientist, investigating and cataloging many varieties of sexuality, not just homosexuality. He coined the word “transvestism,” for example. Although he preferred to project himself as an objective researcher and scientist, Hirschfeld himself was gay and a transvestite, and participated in the gay subculture of Germany. For these activities he gained the epithet “Tante Magnesia” – “Auntie Magnesia.”
In 1919, under the more liberal atmosphere of the newly founded Weimar Republic, Hirschfeld opened the Institut für Sexualwissenschaft (Institute for Sexual Research) in Berlin. His Institut housed his immense library on sex and provided educational services and medical consultations. People from around Europe visited the Institut to gain a clearer understanding of their sexuality. Christopher Isherwood writes about his and Auden’s visit to the Institut in his book Christopher and His Kind. The Institut also housed the Museum of Sex, an educational resource for the public which is reported to have been visited by school classes. The Institut and Hirschfeld’s work there is depicted in the documentary film The Einstein of Sex.
When the Nazis took power, one of their first actions, on May 6, 1933, was to destroy the Institut and burn the library. The press-library pictures & archival newsreel film of Nazi book-burnings seen today are usually pictures of Hirschfeld’s library ablaze. Fortuitously, at that time Hirschfeld was away from Germany on a world speaking tour. He never returned to Germany, dying in exile in Nice in 1935.