Everyone who has ever picked up a pen with the thought of writing anything at all, be it novel, article, or short story, has heard the adage 'write what you know?' Suddenly the world seems like a huge place and that your boundaries in it are pretty tight. After all, how much scope does the average person have?
Plenty really. Write what you know is a great concept--but don't let it bind you into the feeling that you are trapped inside a small box. One thing is apparent that the reason it takes so long for some new writers to become proficient at their craft beyond the nuts and bolts technicalities that everyone needs to control, is that younger writers often lack life experience.
Write what you know does not necessarily mean what you have a degree in. It means what you've experienced. Sometimes it means what you have in-depth knowledge of. It's hard to write about race car drivers if you've never been one. Not impossible mind you: Good reasearch goes a long way to making a believable story--but it's harder than if you have lived in that circle of life either as a driver, or related to one. It's hard for someone who has grown up in the city and never spent a day around a horse, much less worked with racehorses to write a story like 'Seabiscuit'--but it can be done with proper, long term research. What's really hard to write about is the emotional aspects of certain situations if you've never lived through them. It's pretty hard to write about a battered wife if you've never been one. You may get the facts right, but you'll have a hard time identifying with the character and will probably be off on the emotional issues surrounding such events.
So write what you know in my mind becomes more of a write what you've experienced. Emotional cues are harder to connect to than factual ones. You can research and learn facts, it's harder to research and learn emotional responses to varying situations. As you live life and experience more and more varying degrees of emotional events you will begin to grow your 'knowledge' as well. You may still not be able to write about battered spouses (lets hope not anyway), but you will be able to identify more with the frustrations of many varying aspects that effect most of us on a daily basis, and you just may find out you know more than you thought.