And so I discovered that his father, Karl (1868-1948) was a psychiatrist. And because at the time I worked at the Royal Society of Medicine Library in London, which was (and is) a research library of historical record which kept all sorts of things, and we had Index Medicus in print and Medline on CD Rom, I could find material about him and then find the actual articles.
Karl seemed to have been a psychiatrist of some note. People published Festschriften in his honour when he reached significant ages (and we had them, in German). But what did he know about what his family were doing? Two sons and two sons in law were executed by the Nazis. I seem to remember reading in one biography of Dietrich that Dietrich had used Karl's car to transport material to use in an attempt on Hitler's life. And what did he think of the Nazis' policy of sterilisation? Was he involved? I seem to remember material that indicated he was. But why?
I had at the time written an article about Jean-Paul Marat, the French revolutionary, who was a doctor, and meant to write something about Karl Bonhoeffer. But other things happened instead, and I never did. Perhaps this is it!
Anyway, I have just looked again for material about Karl, using the University of Leicester Library search, and found a new thing, and a thing I am not sure I have found before.
The new thing comes from a symposium about Karl's role in Nazism, and is an article by Hanfried Helmchen published in German this year in Der Nervenarzt, and asking about his position on sterilisation (1). And a short piece in the American Journal of Psychiatry in 1944 (2), which seems to be a reprint of a Lancet obituary, reporting that Karl had died the previous year at the age of 75. In fact, he did not die until 1948, which leaves me wondering where that report came from and why.
I hope this is a work in progress!
(1) Helmchen H. Bonhoeffers Position zur Sterilisation psychisch Kranker. Nervenarzt 2015;86:77-84. doi: 10.1007/s00115-013-3999-x
(2) Karl Bonhoeffer. Am J Psychiatr 1944;101:127-8.