Fischer's ideas informed the Nuremberg Laws of 1935 which served to justify the Nazi Party's belief in German racial superiority to other "races", and especially the Jews.

He studied the Basters, offspring of German or Boer men and Black African (Khoekhoe) women in that area.

[6][7] As a precursor to his experiments on Jews in Nazi Germany, he collected bones and skulls for his studies, in part from medical experimentation on African prisoners of war in Namibia during the Herero and Nama genocide.

Adolf Hitler read his two-volume work, Principles of Human Heredity and Race Hygiene (first published in 1921 and co-written by Erwin Baur and Fritz Lenz) while incarcerated in 1923 and used its ideas in Mein Kampf.

After directing the Kaiser Wilhelm Institute of Anthropology, Human Heredity, and Eugenics, he was succeeded by Otmar Freiherr von Verschuer, who tutored Josef Mengele when he was active at Auschwitz.

In 1933, Fischer signed the Vow of allegiance of the Professors of the German Universities and High-Schools to Adolf Hitler and the National Socialistic State.

[19][20][21] In 1944, Fischer intervened in an attempt to get his friend Martin Heidegger, the Nazi philosopher, released from service in the Volkssturm militia.