Writing for an Audience

I've been doing several university classes that are mainly based on personal opinion (politics of the education system and such), and there have been a lot of complaints about the marking of assignments not being the fairest. I've also been called a 'sell out' because I'm giving the marker exactly what he wants to hear... just because I'm getting top marks for it.

But, this got me thinking, when writing are writers aware of what their audiecne wants? I can say that when I write, I write for myself... but, I also induldge in a lot of YA's so I know exactly what's out there and what's interesting, so I guess I know what the audience wants.

How about you? Do you write what for an audience or do you hope there'll be an audience for when you finish?