A large part of my job (especially in the last couple of months) is processing new books.
Now non-fiction is easy enough to categorise. The Dewey Decimal system is largely sensible (even if there are odd bits I don't quite agree with) and the books come with numbers already decided.
Fiction however can be more of a minefield. Where I work we put all our fiction in alphabetical order by author's surname, this is nicely simple and lacking in controversy. However some libraries (and I think almost all bookshops) organise by genre as well, and here is where things get tricky.
I reviewed The Windup Girl by Paolo Bacigalupi last month. I think I made it pretty clear that it was Science Fiction. I also recommended it be bought for work as there was a waiting list on it. When the book arrived I noticed that the genre label provided by the distributor was Thriller, not Science Fiction. This got me thinking, as I have many times before, about how genre is decided.
I think one way of looking at the question is by examining content and tone.
Let us take a well-known example:
The Time Traveler's Wife by Audrey Niffenegger is a mainstream bestseller, but it isn't completely grounded in the possible. It's about a relationship between two people, one of whom has a genetic disorder that causes him to jump around in time.
Time travel = SF, surely?
Well yes, time travel is currently impossible thus putting it into the realm of SF. However there's no time machine, wormhole or time dilation, nothing scientific or technological. The book is about the relationship between the characters and the lives they lead given the unusual circumstances, in this respect it's a very grounded, realistic book.
It's important to note that huge numbers of people who read and enjoyed this book are not SF fans, and would probably say they do not like SF and would not read an SF book. Fair enough, people like what they like, but if they enjoyed The Time Traveler's Wife then surely they can't be against everything science fictional.
In these cases I think that the tone is the key. It doesn't feel like SF. It's written in a more mainstream/literary/romantic way, so people can read it without identifying it as SF, despite the major time travel element.
Going back to The Windup Girl.
If a book is an action-packed and absorbing political thriller set in an imagined future, is it Science Fiction or Thriller? It won a Locus, Hugo and Nebula, which would suggest SF, but I'm not aware that there are many awards for Thrillers.
In fact the Thriller label seems to be applied to a wide variety of books, so I'm not entirely sure how to easily describe it. I mean 'thrilling' is the obvious descriptor, but you have excitement and action in other genres, so that doesn't quite seem right. Besides what a person finds thrilling is very subjective. However I think it's safe to say that danger, excitement, a race-against-time, and plenty of peril are an important part of any Thriller, and The Windup Girl contains all those things.
So perhaps the next question is what is more likely to get taken off the shelf?
I suspect that the answer is a book labelled as a Thriller.
As I've just pointed out Thriller is actually quite a general genre, whereas sci-fi is not. So if you've a book that could be two genres you might as well put it under the less niche one.
I think this is why The Time Traveler's Wife is usually in general fiction, rather than SF. It'll find an audience in both, but there likely to be more people looking in the former.
This is also why I've left the Thriller label on The Windup Girl at work. Either way it's going to be filed under B for Bacigalupi -because that's what we do. Perhaps a few more people will pick it up if it has a little gun on the spine, rather than a little Saturn-like planet, but then again plenty of people won't look at either.
Besides, if someone picks it up expecting a modern, realistic thriller they'll probably notice the zeppelin and the mammoth (technically a megodont) on the front cover.