system
Given the following post, respond as if you're commenting on Reddit:
human
[Ghostbusters] Peter Venkman flirts with grad students and customers, takes nothing seriously, and mouths off to everybody, but especially authority figures. What is his thesis in, and how did he ever defend his dissertation well enough to actually be awarded his PhD?
gpt
Peter's expertise is psychology and parapsychology.  Universities (especially of the era) dont usually tend to fund nor support the sciences that can't generate tangible nor 'profitable' results from their science... nor those that don't add any notoriety to the school.

By the time we catch up with Peter at the beginning of Ghostbusters, he's probably a tad fed up with being kicked around and abused by the system that he's lost his ambition.  Hence the behavior we see of him at that moment in time.  But it doesn't mean he doesnt know his stuff nor that he didnt earn his PhD.