Can I defer a PhD program if I have accepted another?

[misslitty]

So I have accepted a PhD offer at a very prestigious institution, but I am concerned about the PI that I will be working under.  He has exhibited a lot of red flags, and I am under the impression he is not a good mentor, bordering on labor abuse.  Is it possible to defer my other PhD admissions for a year, granting me time to see whether my concerns are valid, or if the relationship can work?  If so, how should I phrase this to the other institutions? 

[Bird Vision]

It sounds like you should un-accept the prestigious offer. It's going to be awkward and uncomfortable, but it happens. Either that, or decline the school you want to defer.

Deferment is allowed to compensate for unforeseen personal circumstances. If you end up not wanting to stay at the prestigious school, you can probably get reaccepted elsewhere.