What are the arguments in favour of making the PhD defense open or closed?

Question

While it appears to be standard practice in all the examples I've heard of for examining committees to conduct their deliberation in camera, I've noticed that the rules regarding who can attend what parts of a PhD defense vary significantly around the world and even between institutions within the same country.

What are the justifications/arguments for greater/lesser restrictions on who can attend which parts of the defense?

There doesn't seem to be a question about this on SE already, though I've seen some answers on other forums. The answers to this related but not applicable question do provide some insight. I found a blog post that seems to indicate that in the US the method by which defenses are conducted is a legal matter.

Example

At my institution (Canadian, public university) the list of people who can attend what we call the Final Oral Examination, or colloquially the PhD Defense, is highly restricted. There are multiple steps to completing the PhD degree requirements here:

1. A Department Defense, where the PhD candidate gives an open seminar at their department. The examination committee does not need to attend this one. Usually it is just attended by your supervisors and some fellow grad students. Supervisory committees can, and often do, vote to skip this one entirely.
2. A Final Oral Examination, which is conducted entirely behind closed doors. The examination has three parts: the presentation period (20 minutes), the question period (60-120 minutes), and the deliberation period (10-30 minutes). The candidate leaves the room during the deliberation period, and then returns for the committee to give their decision (pass/fail).

The only people, other than the candidate, allowed to attend any part the Final Oral Examination are the members of the examining committee, a chairperson appointed by the faculty, and any member of the faculty at the university (upon the invitation of the chairperson). The one exception are "qualified observers" which must be approved in advance by writing a letter (in hard copy) to the Vice Dean of Graduate Studies. The deliberation period is attended only by the committee and the chair.

My understanding is that this is highly restricted compared to other institutions, even within Canada. On the other hand, when I defended my Master's degree at another Canadian institution, I invited my best friend to attend (he wasn't even a student at the university). These other attendees were allowed to be present for everything except for the deliberation period. My understanding is that this is very open compared to other institutions.

Answer 1

I think these things are mostly determined by tradition rather than specific arguments for or against. There's a lot of variation by program even just at my own institution.

Reasons to be public would include that universities are ultimately intended as institutions that benefit the public - a thesis defense is an opportunity to share the work done by students at the institution with the public. It's also an opportunity for the student to share their work.

More personal reasons include a recognition that successful graduate students are supported by more than just their PI - it's an opportunity for family and friends to both show support for the defending student and get recognition for their support. It seems common at least in the US for students to celebrate after their defense - that's usually a more important moment than any formal school-wide graduation ceremony, though that also differs by program, but inviting the people who you'd like to celebrate to the defense seems a natural extension - come for the event, stay for the party.

Public defenses also benefit other students, who can see what a defense is like before they have their own defense, and helps to (lightly) standardize the process a bit - by making the defense open, it would be noticeable to others if a particular defense was conducted unusually.

In my experience, public defenses are always followed by a private session, which likely includes both questions directed at the student and deliberation among the thesis committee. The private session allows for honest sharing of opinions among the committee and it takes takes some pressure off the student to perform or be criticized openly in public. I think a closed defense is simply recognizing that the most concrete purpose of a defense - determining whether a student has passed - is this private examination, recognizing that the other benefits I described above can just as easily be achieved through other means. There's no need for a student to present their work only at their defense, for example. There are other ceremonial opportunities to recognize and be recognized by family and friends, like in a formal graduation ceremony after the defense.

I think there is also perhaps more of a stress on the rigor of the process in a closed defense - if a student doesn't pass, suddenly the open defense seems to be quite an awkward affair.