As a writer of science fiction and fantasy, and on behalf of all the variations and sub-genres such as urban fantasy, alternate history and steampunk which collectively make up "speculative fiction", I'd argue that genre fiction is different from literary fiction.

Whether it's dealing with rayguns and rocket ships, swords, sorcery or fur and fangbangers, speculative fiction's unifying, identifying characteristic is that it doesn't attempt to mimic real life in the way that literary fiction does. It stands apart from the world we know. It takes us away to an entirely secondary realm, be that Middle Earth or Westeros, or to an alternate present where vampires and werewolves really do exist and you ring 666 to report a supernatural crime.

Read science fiction and you can visit a near-future United Kingdom where advances in bio-sciences see mega-corporations using genetic engineering to exploit the human genome for corporate profit. You can see mankind terraforming Mars or can step into a far future where humanity has colonised the stars, leaving an excluded, remnant population on Earth trying to understand their place in this expanded universe.

I've read all those stories recently and as it always has, that distinctive unfamiliarity made me read with closer attention. I wouldn't go so far as saying "familiarity breeds contempt" with literary fiction, but familiarity can certainly breed speed-reading. The reader's unconscious mind latches on to familiar elements and fills in the rest. If you're reading a novel set in Manchester, even if you've never been there, you have a mental image of "Manchesterness" from other cities you've visited and what you've seen on the television.

Peter Jackson's best efforts notwithstanding, no one's ever been to Middle Earth. Speculative fiction prompts the reader to pay so much more attention, looking for the details that make sense of this strange world. Reading speculative fiction isn't arriving in Manchester. It's finding yourself in Outer Mongolia with no help from Lonely Planet or a Rough Guide.

Which is why, done well, speculative fiction can be considerably harder to write than literary fiction. I can tell you from experience, as an author, as a reviewer, and after spending two years as a judge for the Arthur C Clarke Award and reading around 150 novels, that when readers are paying that much close attention to every hint and clue, the writer needs to have their internal logic, consistency of character and scene-setting absolutely nailed down. Readers have to be convinced that this unfamiliar world is solidly real if they're ever going to suspend disbelief and accept the unreal, whether that's magic and dragons or faster-than-light travel.

You absolutely cannot obscure underlying weakness with waffle. Otherwise the emails will arrive, picking up on discrepancies. Not just for the sake of point-scoring or nitpicking but because fans become so engaged with imaginary worlds and so passionate about their characters.

That passion, so easily mocked by laughing at Trekkies and Whovians, is another thing that distinguishes SF and fantasy from literary fiction. Mocking that passion is missing a key aspect of speculative fiction. By drawing readers in large numbers, contemporary fantasy becomes a platform to debate key, current social and political challenges, while science fiction continues to explore the impact of technological developments, for good and ill, before we have to tackle these things in reality.

Speculative fiction may not mimic real life but it uses its magic mirror to reflect on the world around us. It's a fundamentally outward-looking genre, in direct contrast to literary fiction, which looks inward to explore the human condition.

Setting a story in another place or another time enables speculative fiction to explore ideas that literary fiction might really struggle with. I'm interested in divided societies; my father's Irish, my mother's English and the versions of Irish history I learned at my Granny McKenna's knee and at a girls' grammar school in Dorset in the 1970s were pretty radically different. I have friends who've lived and worked in Yugoslavia, as it was, and later in Croatia and Bosnia. I know diplomats who've had dealings with Israelis and Palestinians. A literary novelist dealing with any of those intractable, complex conflicts faces countless challenges and pitfalls.

Write a fantasy novel centered on a fractious, fractured country, where arrogant aristocrats pursue their ambition heedless of ordinary people's suffering and you can explore the rights and responsibilities of power, the uses and abuses of privilege and the importance of people of every class getting involved in managing their own destiny. With turnout at elections steadily dropping and abuses by the rich and powerful going unpunished because they're financing the political classes, getting people to think about the world around them is important.

Inspiring the next generation of voters to engage with the political process is vital. Science fiction and fantasy can do this because speculative fiction gets young people reading. It's been the mainstay of children's literature from Edith Nesbit through CS Lewis to Diana Wynne Jones, Philip Pullman and Francis Hardinge. It's become a dominant force in popular culture from Battlestar Galactica and Game of Thrones on TV to the Hunger Games films and the new Captain America.

However, this is a double-edged sword. The common assumption that "kids' stuff" or "commercially popular" means simplistic or inferior helps perpetuate the prejudice against science fiction and fantasy. It's no wonder that writers like Margaret Atwood put so much effort into distancing themselves from what they see as a damaging association. I wish she didn't do it – but I can see why she does.

It's also very amusing to watch the contortions of literary critics faced with talented writers like the late lamented Iain Banks and Joanne Harris, who are equally adept in literary and speculative fiction and refuse to apologise for or justify what they write. And if the definitive characteristic of literary fiction is sublime prose, then at his peak Terry Pratchett is surely the finest prose stylist writing today. So this is where we get terms like counter-factual and magical realism, to save reviewers from sullying their copy with words like SF and fantasy.

Challenge people to justify that disdain and you'll almost always find out they don't actually read current speculative fiction. They read The Lord of the Rings at school and lost the will to live somewhere around Tom Bombadil. Frankly, that's understandable and I say that as a Tolkien fan. But to reject an entire genre on that basis? That's like reading Murder on the Orient Express as a teenager, deciding the solution is preposterous and dismissing all crime fiction. Such prejudice makes no sense. And worse, it cuts a reader off from some of the most challenging, most immersive contemporary fiction.

So yes, as far as I am concerned, the speculative fiction genre is different to literary fiction. By celebrating its distinctive strengths we will come to recognise the weakness of knee-jerk prejudice against it.

Of course, accepting this difference doesn't mean is we need to choose between reading one style of writing or the other. A fully rounded literary life means reading across the full spectrum of fiction.

• Juliet McKenna's works include the fantasy series The Hadrumal Crisis, The Chronicles of the Lescari Revolution, and The Tales of Einarinn

• This is an edited version of a speech given at an Oxford Literary Festival debate on the motion "Genre fiction is no different from literary fiction"