A Ph.D. in Psychology has been helpful to me in my writing -- in some expected ways, in others perhaps not as expected.
As you'd might expect, I did learn a good bit about different ways of being in the word, and about the ways various levels of psychopathology can disrupt life. That's pretty helpful from a character development standpoint.
Also expected, because I did a great deal of it, my academic writing was pretty sharp by the time my dissertation was done. Although that doesn't necessarily translate directly to writing fiction, it doesn't hurt.
What I really learned in grad school, and what was helpful in writing, is possibly a bit less expected. I learned to play by other people's rules in the name of achieving a goal. I learned that a Very Big Thing (i.e., a dissertation) can be completed bit by bit if you don't just chuck the whole thing in frustration, thinking there's no way you could possibly complete such a herculean task. I learned that sometimes, those with power to pronounce you "successful" will be looking out for your best interests, and sometimes they won't be.
I suppose the most important thing I learned in grad school which is helpful in writing fiction, is that there are many methods of getting-the-job-done, and that once you find the way that works for you, stick with it, even if it doesn't mirror the technique of those around you.
~STS~