Horkheimer assumed full Institute directorship in October 1930 regardless of Grünberg’s recovery.

Evidence

In October 1930 Friedrich Pollock (who had been Felix Weil’s executive agent since 1925), representing the board of the Society for Social Research, and Max Horkheimer, who two months before had been appointed Professor of Social Philosophy, signed a contract. Paragraph 3 read: Professor Horkheimer becomes director of the Institute as of today’s date. Should Professor Grünberg unexpectedly recover from his serious illness to the extent that he is able to return to work as director, Professor Horkheimer will try to reach an understanding with him on the division of the director’s duties. Even in the latter case, Professor Horkheimer will reassume sole direction of the Institute on 10 February 1932 at the latest. — Rolf Wiggershaus, The Frankfurt School, 140