It was at Hamburg that he studied successfully for his habilitation (higher academic qualification) while working as a research assistant for Moritz Liepmann [de].
He served on the senate (governing council) of the Nazis' Tropical Medicines Academy ("Kolonialärztliche Akademie der NSDAP").
Sources are imprecise on the details and extent of Sieverts' involvement with the Nazi regime, but he was briefly interned after the war at the Neuengamme internment camp established in the British occupation zone to hold people suspected of involvement in Nazi crimes in the Hamburg area: he was released only in 1946.
[5] Later on, Sieverts was a member of the expert committee on youth justice which had a decisive influence over the 1953 reform of the Juvenile Courts Act.
He was temporarily president of the regional High Court in Hamburg, and in 1960 he chaired the Working Group on Criminal Law reform.