Diederik Stapel on November 9, 2011 voluntarily surrendered his doctorate at the University of Amsterdam. The certificate he received in 1997 after his promotion is now back in possession of the university.
Stack has declared in writing to waive his Ph.D. because he believes that his behavior in recent years does not fit with the duties associated with the doctorate. Stack graduated in 1997 from the University of Amsterdam. He worked there from 1993 to 1999. He then worked in Groningen and Tilburg. On October 31 the Committee recommended Levelt, who investigated the scientific integrity of the work of Stack, the University of Amsterdam to investigate a Ph.D. may be withdrawn.