[1] The couple had three children: daughter Hedwig (1899–1996, physician), and sons Karl (1903–1940, Gestalt psychologist) and Wolfgang (1909–1942, journalist and film critic).

In 1907, the family moved to Stuttgart from where Hermann Duncker toured the country as an itinerant teacher for workers' education.

Together with Karl Liebknecht, Rosa Luxemburg and Clara Zetkin they were among the founders of the Spartacus League that became the Communist Party of Germany (KPD) during the German Revolution of 1918–1919.

[1] After the Nazis' seizure of power, Duncker, like most communist leaders, was taken into "protective custody" in February 1933, but he was released in November of the same year.

Duncker also fell out with the KPD's Moscow leadership over the Molotov–Ribbentrop Pact between the Soviet Union and Nazi Germany, which he strongly opposed.

[1] Käte Duncker had fled to the United States, living with their son Karl who suffered from depression and committed suicide in 1940.

He joined the ruling Socialist Unity Party of Germany (SED; created in April 1946 by the forced merger of KPD and SPD).