Back when I was in college, I ended up in a lot of writing classes. Shock! A Creative Writing Major in a writing class? How novel! (Ha. I made a pun.)
One of the things we were told over and over again was that we should "write what you know." The YouTube channel Overly Sarcastic Productions made a really good Trope Talk episode about this, and it got me to thinking about what it actually means.
Most folk seem to interpret "write what you know" as more or less "write what you have personal experience with." But if that were a hard and fast rule, then no sci-fi or fantasy novel ever written would feel genuine or even interesting because the author was pulling details out of thin air, right?
Obviously, that isn't the case. We have sci-fi and fantasy books by people who've never even left their home country, and yet they're compelling and genuine in ways that most nonfiction books never even touch. So what's with this "write what you know" nonsense?
The fact is that I'm not an expert on the origin of the saying or even the recent trends in literature that might have led to its widespread use. But I have an educated guess.
Someone, somewhere, at some time, got tired of reading articles, books, serials in which the author just made stuff up and pretended that was the gospel truth. Mystery writers that didn't know the first thing about murder investigations. Romance authors that had never been in a real relationship that functioned in anything like a healthy way. Spiritualists that claimed to have answers to being cold at night and unnecessary sneezes.
That person probably turned around and started teaching the new generation of writers that you're supposed to write what you know - meaning that passing off a shot in the dark or even an educated guess as fact was very much not an okay thing to do.
Now that in itself is simply a guess, but I think it's a relatively good one, because it gets to the heart of what "writing what you know" means in practice. Not to restrict yourself to purely experiential knowledge, but rather to ensure that if you're going to represent a situation, character, or message in writing, you ought to be reasonably confident you understand it before you publish it.
In short: do your research.
Now, if you don't mind terribly, I'm going to go do some research into different marriage customs and what gave rise to them. :)