I was researching for a different story and stumbled onto a paragraph about the eugenics movement in early 20th century New Zealand, which I had previously not known existed. A few days later, I dropped everything else and started planning and researching for this story. I tend to find that’s the best thing to do when writing: if an idea stands out, follow it wherever it takes you.

The most difficult thing about researching eugenics in New Zealand was that relatively little has been written about it. There’s no single book you can read to get the whole story. I had to piece it together from many different sources – from academic articles to 1930s newspaper stories to the records of parliamentary debates. This meant that for the first year and a half, I was constantly replanning and rewriting. I would find a new piece of information and have to completely change the story to accommodate it. For example, I had been working on the book for almost a year before I found out that New Zealand had come close to passing a eugenic sterilisation law in 1928. None of the other sources I’d read had even mentioned that.

Beyond the topic of eugenics in New Zealand, I had to research the international eugenics movement, the Great Depression, and all the other big ideas covered in the book. I read Darwin and Plato and Machiavelli – all fascinating, but not the easiest books to read. Even the details of everyday life in the ’30s could be a challenge. It’s easy enough to learn about the big events in history or what prominent figures said and did. It’s a lot harder to get a sense of the lives of ordinary people, especially those who were young, working class, or female – and some of my characters are all three.