I've always been told to write what I know. Teachers, friends, books have always said "What topic have you studied? That should be what your writing focuses on."
Not to break with tradition, but I disagree. Completely.
Writing what you know is an acceptable formula for textbooks and nonfiction; in those areas, it is essential to know the material down to the minute detail. Otherwise, the entire work loses credibility. In fiction, however, this is simply not the case. To limit one's writing to what they know eliminates countless plotlines and innumerable stories.
Personally, I know very little about being a mother. I'm a male; I also have no children. Yet, "Dulce et Decorum" is one of my favorite pieces I've ever written. In it, a mother loses her only son to the horrors of the Wars against Napoleon.
J. A. Konrath, one of the most successful indie writers out there, broke into traditional publishing with his Jack Daniels books. The protagonist in that series is a female detective. Konrath is a male and has never worked in law enforcement. How then has Konrath sold thousands of copies of his books? He doesn't really know much about being a woman and a detective...Or does he?
Rather than writing what we know, fiction demands something different, something better:
Write from your Passion.
In my case, I feel passionately about the personal loss in war. A mother's grief is far more important to me than the faceless statistics of a battle won or lost. Seeing how conflict wrecks the lives of a single family far outweighs cries for a national struggle through bloodshed. In brief, I write to display that passion to others. So, my writing has often focused on warfare in different ages and the human suffering behind those numbers. I won't speak to Konrath's passions, but he has written extensively on his blog about passion for storylines. To mince his words, if a writer doesn't feel passionate about their plot, there's not a jot of a chance that a reader will.
When I write, I don't necessary know what it's like to be a mother, or a grieving lover; I have never lost someone to war. Rather, I write with passion for a subject and do research to supplement what I don't know. Writer's who start with what they know can produce facts and details quickly and efficiently. But writers who write from their passions produce stories that audience will want to enjoy; if the reader can sense the emotion and life breathed into the piece, they are more likely to connect with the work.
Write from what moves you; the knowledge can follow.