Write what you know” may be trumpeted a lot, but you can’t apply it literally. Otherwise you end up with nothing but stories about grad students struggling with collegiate life. That’s fine if student life is what you really want to write about, but some institutions hand off that sort of expectation like a weighty anchor, effectively sinking poor students so deep that they can’t surface to master the strokes they’re most interested in. Write what you know shouldn’t mean only what you have personally experienced, it should also mean to write what you love. If you want to write about gangsters or spaceships and you’ve done the research, then you can be said to know it. If you need to do more research to better know what you love, make it so. You can then apply your personal knowledge of, say, human relationships, to inform your topic.