John Marrs reports the novel’s inspiration came from others’ conversations about the experience of “love at first sight.” Fascinated by those stories, he wondered both what underlies the experience and what would happen if everyone were guaranteed such a love (Sobel, Ariel. “What Happens When You Can Find Your Soulmate From a Blood Test?” The Advocate, 13 February 2018).

The idea that everyone has one perfect soulmate dates to antiquity. It appears in the Egyptian Book of the Dead (2000 BCE) in Babylonian mythology and the Judeo-Christian creation story in Genesis. The idea of the soulmate suffuses folktales, modern romance, the music we listen to, and it is introduced to children as soon as they are old enough to watch the classic Disney movies.

In Plato’s Symposium (360 BCE), the poet and comic playwright Aristophanes suggests that humans were originally a hermaphroditic species with eight limbs, two heads, and one soul. These creatures offended the gods, and Zeus split them in two, condemning each half to wander the earth in search of the missing part of itself:

And when one of them meets with his other half, the actual half of himself […] the pair are lost in an amazement of love and friendship and intimacy, and one will not be out of the other’s sight, as I may say, even for a moment (Plato.