Genre is a tricky word. Arguably it's no more than a label for something which could exist perfectly well without it. So why is genre considered so important?
There are several reasons. Firstly, libraries and bookshops – whether online or bricks and mortar – have to have some way to organise the books on their shelves, and it has to be such that the average reader, browsing the shelves, can easily find what they are looking for. That calls for a method of organisation which is pretty much universal.
Secondly, people just love to categorise things. Find me something which isn't categorised and I'll find you someone who wants to fix that. On the surface this desire seems gratuitous, but there is an underlying reason for it. Our minds naturally organise things so we have quick access to them in our memory and can better understand them.
As readers, when we find a book we like, we often want to read more of the 'same kind of thing'. Genre, at least in theory, allows the reader to choose books that are likely to be to their taste. If a reader likes stories about aliens and space travel they might be disappointed by a book which is a love story – then again they might be pleasantly surprised.
The enjoyment of a book is highly subjective, and two big factors influencing a reader's potential enjoyment are the genre and the author.
So does that mean a fiction author should stick to a chosen genre?
Most authors do. That doesn't, however, mean it's necessary. It may simply be a preference on the part of a large number of authors. Interestingly, those who do write in more than one genre frequently use different author names for the different genres. The much loved Ian Banks comes to mind. When writing science fiction he put an M in to be Ian M Banks. Many others use completely different pen-names.
The question is what goes wrong if an author writes in multiple fiction genres under the same name?
I think the biggest risk is with readers – particularly in a world in which a given author name is usually associated with a particular genre. The last thing an author wants is to let their readers have a hard time figuring out whether to read their next book. Worse still, if an author has a strong following, they could end up with disappointed readers if there's confusion over the genre of the next book. No author wants that.
Personally I like Ian Banks' approach. An Accident of Birth is dystopia, which comes under science fiction. If I write outside of the science fiction genre I shall use a variant of my name to differentiate between the two.
That's not going to be an issue in the near future, though. My plan was, until recently, to write a crime thriller next. However, that plan has changed. At the moment I'm really fired up about a science fiction space opera, so that will be next.