I was reading a profile of Tabitha King in Writer's Digest the other day, and she spoke about that old writers' adage, "Write what you know."
She said, "My feeling is 'know what you write.'"
I absolutely agree. After all, I never lived in the Middle Ages, yet I've written 25 books and novellas set in that time period. I've never lived in the Dark Ages or Restoration, Regency and Victorian England, either, yet I've also written books set in those times. Heck, I'd never even been to England until I'd set several books there.
So how did I have the nerve to do it?
Well, my readers haven't been there, either. And I research. I want to know what I'm describing, whether it's clothes or food or social customs. I don't know everything of course, but I try very hard to be correct about what I include in my books.
I also write what I know emotionally. My family moved a few times in my childhood, so I know the feelings of being new and trying to fit in (or not). That's why I tend to write stories of "the stranger rode into town" ilk. That's also why I think "home" is not a place so much as a feeling of security and acceptance.
I also understand the stoic, stiff-upper-lip response to troubles. That's my folks. And that's why my heroes often fall into that category -- because I know full well that just because somebody isn't weeping or wailing doesn't mean they aren't devastated.
So by all means, write what you know -- but it shouldn't limit creativity. You can learn what you don't know, and I bet most people already know a lot more than they think.