Both the theoretical and the practical work at the Institute bore the stamp of Hirschfeld’s thinking. From 1896 up to the Institute’s foundation, Hirschfeld extended his field of work to embrace not only the specialized topic of homosexuality but all aspects of sexual science.
His scientific point of departure was built on the the theory of the “naturalness” of the third sex which, in turn, went back to Karl Heinrich Ulrichs (1825-1895) 1 2 3 . Hirschfeld was convinced that both the social and legal views of same-sex behaviour could be altered by means of scientific reasoning only. He established his theory of sexual transitions, developed a concept of sexual science and set out a sexual theory oriented toward the study of reflexes and evolutionary biology. The elaboration of his theoretical approach culminated in the publication of the three-volume “Sexual Pathology” (1919), coinciding with the Institute’s foundation.
To substantiate the natural character of sex and gender, he incorporated, until 1910, findings yielded by genetics into his concept [4 5] . As research into hormones and ductless glands emerged before World War I, sexual hormones came to constitute the foundations of his theoretical structure. The full extent of sex and gender variability was accounted for one-sidedly by means of the “glandular orchestra”  .
By the mid-20s, it had practically become impossible to sustain Hirschfeld’s simple and universal interpretation of the effects of sexual hormones. Consequently, scientific production at the Institute fell sharply, with work, from this time on, focusing on therapy, counselling, sex education and sexual reform.