The below insights were organized from a text-based conversation with friends of mine who were kind enough to encourage me to ramble about book genres, despite that they don’t really read themselves. They correctly observed that a lot of modern stories blur the line between science fiction and fantasy, and asked, “Well, then why does Barnes & Noble bother having separate shelves?”
Genre fiction, as a concept, exists as a categorization scheme because of the publishing industry and marketing needs. The way we think of genre categories grew out of mass market fiction. So they’re organic categories popularly defined by recent tradition. On top of that, they vary from language market to market— For example, you have the Spanish picaresca, the Chinese wuxia, and the Japanese isekai.
When it comes to contemporary science fiction and fantasy, you could characterize them as having one key difference: Science fiction, ostensibly, does not explore the supernatural. Both genres may feature imaginary worlds where fantastical creations/abilities/phenomena exist. But science fiction is “supposed to be possible.”
However, clearly a lot of sci-fi stuff isn’t possible. Or probably isn’t possible. There are many magical, fantastical elements that are staples of the genre because of classic sci-fi works. Simply put, popular perception of what may be possible—of what falls under “the realm of science”—has shifted over time. Psychic powers are a good example of a classic genre staple. Though nowadays, outside of superhero stories, it’s become rare to find a sci-fi where the human characters have psychic powers. They’re fair game for aliens and magic-wielding characters, but the idea of hidden, superhuman mental abilities waiting to be unlocked by technology has largely passed out of the popular imagination.
Different tropes associated with genres fall in and out of favor. The categories are loose, and they’re largely defined by what has come before. This is why what really matters when choosing a genre for your book is the expectation that you’re setting for today’s readers. For example: I remember Xiran Jay Zhao saying on social media that her Iron Widow falls under a particular Chinese genre fiction category. But it was published in Canada, so it’s a sci-fi. While a lot of this seems arbitrary if you’re thinking about novels as discrete works, the rules are well-defined in the context of precedence and marketing.
When an author’s in doubt about what genre their story falls into, they can be tempted to comb through their plot elements, tallying up robots in the sci-fi column, a love interest in the romance column, ghosts in the supernatural column, etc. This is the logic that leads to people asserting that they’ve written a sci-fi romantasy horror mashup that “breaks genre bounds,” which is a confusing answer when someone asks what they can expect from your book. Instead, the author should be comparing their story to other recently published stories, and also, critically, thinking about the narrative structure. Certain kinds of plots are more typical of fantasy than sci-fi, and vice versa. The romance genre is defined by the plot above all: If there’s no happily ever after, it’s not a romance.
Every so often, someone publishes a story that becomes iconic as a new (sub)genre definer. But it won’t get that status until it’s been widely read enough to become a point of comparison and inspiration for many people, so it’s going to start with a more generic label. Vampires originated from gothic horror; casting them as love interests is a relatively recent trend. Even by the time Twilight came out, you’d see it stocked in the horror section as often as the romance section. It took years and tons of imitations for the idea of a “vampire/paranormal romance” to become ubiquitous. Meanwhile, the She-Ra franchise has spaceships, AI, aliens, and holograms, but the plot is about a chosen one wielding a magic sword to help princesses save their realm from an evil despot. So I bet it would be shelved as fantasy.