Are fantasy series more likely to expand, unplanned, than science fiction?Actually, you'd be surprised to know that science fiction is the more likely genre to have unplanned expansions. Unlike fantasy, science fiction is not as readily susceptible to series-itis. This has quite a lot to do with the kinds of stories being told, the history of the genre, and loads of other factors which won't be uttered here due to space constraints. But I'll talk about some of the important bits here.
There's a fundamental difference between how fantasy and science fiction novels are sold. Due to an obsessive need for long, epic trilogies, stand alone fantasy novels are relatively rare in comparison to series--in the eyes of the public, at least. There are, of course, authors who thrive on stand alone books, but the series is the name of the game. Typically fantasy authors write a duology, trilogy, quintet, etc. and sell it to the publisher either one piece at a time (such as Patrick Rothfuss seems to have done) or in bulk. Some of them succeed and are published in full, and others do not.
Science fiction, however, is a lonely road. Few science fiction authors get published based on the proposition of a series. But where did all those sequels and what not come from? Some are the result of the publisher's request, some due to fans, and others due to perceived unfinished business by the author. Most sequels aren't planned. They might be lingering in the back of the author's mind, but it's not often that a science fiction author actually sits down and plans out a series of science fiction books (a few have, of course, and there are several science fiction series out there, from David Weber to Isaac Asimov).
The differences between the two is important to note, because while fantasy is intensely series-based, it is not expansionary in nature. Science fiction, however, is. It is rather uncommon to hear of a fantasy author deciding to expand a series that was planned to stop at three books. Christopher Paolini is perhaps the highest profile example of a fantasy series expansion, but science fiction is littered with examples of stand alone books expanded either by sequels or longer series--a good example would be the recent addition of C. J. Cherryh's Regenesis, the long-awaited sequel to Cyteen.
Fantasy will continue to be dominated by series, though, and people will typically recognize it for its long, drawn-out epics rather than for its stand alone gems. Science fiction, however, will be the exact opposite, and every so often we'll get an outstanding series to drive a little of that epic feel into the science fiction landscape.
But I'm just one person with one perspective. If you have something to add, or a different opinion, let me know in the comments!
If you have a question about science fiction, fantasy, writing, or anything related you'd like answered here, whether silly or serious, feel free to send it via email to arconna[at]yahoo[dot]com, tweet it via Twitter to @shaunduke, or leave it in the comments here. Questions are always welcome! If you liked this post, consider stumbling, digging, or linking to it!