And why not? There's someone for everyone in crime/mystery - elderly lady sleuths, amateur Palestinian sleuths, professional Belgian sleuths, thoughtful Scottish police officers, embittered Scottish police officers, damaged Irish police officers, weary Scandinavian police officers, former army officer detectives, amateur girl detectives, sad drunk amateur girl detectives. There's even a detective who partners up with a skeleton.

What there doesn't seem to be much of in mystery fiction, however, is female detectives who begin detecting in middle age (two cases in point: Sara Paretsky's V.I. Warshawski was 32 when we began to follow her adventures, while Patricia Cornwell's Kay Scarpetta was 36). Women in the 40-to-60 range don't get much of a showing as main characters in literature generally, so maybe it's no surprise that they don't show up as headliners in the mystery genre.

But it's a surprise if you're a woman in your 40s who is beginning to write mystery novels - which not so long ago I was. And while it's a cliche that people write what they know, when I decided to write a mystery I entered into that cliche wholeheartedly. I started writing mysteries in my 40s - and I made my detectives two women in their 40s. But while I designed my detectives in part to mirror myself, I also chose their age for entirely different reasons.

For all its advances and improvements, contemporary culture remains uncomfortable, not just with middle-aged women, but to an even greater degree with contentedly single middle-aged women - and to an even greater degree than that with childless women who are childless by choice.

I wanted to confront this odd aversion head on, so I made sure that between them my detectives fill all these categories. This decision is a small one, but for me it engaged with a question I've long struggled with, the question of what, intellectually, literature is supposed to do. Should it report on the world as it is, or should it model the world as it might - could, should, would - be?

Recent fiction may not have been filled with middle-aged women, but it has been filled with angry ones. It's jam packed with dystopian models of female repression and with women and men implacably set against one other: look at Naomi Alderman's The Power, Leni Zumas's Red Clocks, Margaret Atwood's The Testaments. As I began to create my fictional world, I was troubled by the question of whether these outpourings of rage, all justified, all earned, are enough.

In one way this literature is unquestionably good: female rage is something else culture has assiduously avoided considering for centuries, and during those centuries women have been subjugated, silenced, used and abused in ways that deserve outpourings of anger.

Nor is this landscape of female repression changing quickly, and there is value in forcing it before readers' eyes. In a recent article for The Guardian about the Staunch Prize, the new award for the best crime/mystery novel that does not feature violence against women, the novelist Kaite Welsh wrote that she wouldn't "sanitise my writing in service of some fictional, feminist utopia ... my work lies in marrying my imagination with the ugly truth". This is an important argument.

But while I don't care about the absence of feminist utopias, I do care about the absence of writing that models the future we want. What will it be like, the world we're striving for where the playing field is level and men and women are just people being people together?

I believe that we can only imagine based on what we've seen, and that it's part of literature's job to help us see what doesn't yet exist but could.

For this reason, although I grounded my novels firmly in the present (well, Death in Paris is set in 2014 and The Books of the Dead in 2016), I made some specific choices about that present. In my books, every person in a powerful position is a woman. Department heads and doctors, sources of knowledge and implacable foes: all are female.

More importantly, as far as I was concerned, no one draws attention to that. It's so normal as to be unworthy of note.

Nor does anyone comment on the fact that both my detectives are childless, and one remains happily unmarried in her mid-40s.

Both of these detectives are heterosexual, but I made that decision so I could give each a husband or boyfriend who sees them as an intellectual and emotional equal, and for whom that equality is also the norm.

The women in my books have and enjoy good sex, and they have and enjoy good conversations - almost none of them about men.

My books are full of oversights and omissions, and I'm far from satisfied with them.

But what I aimed to do, and am still trying to do, is to augment the justifiable depictions of anger, the honest depictions of ongoing brutality and violence against women, with a small model that allows for a morsel of hope.