Once there were only men and women, or very occasionally a hermaphrodite. Nowadays people can define their gender as they please. Gender reassignment treatment is now so sophisticated that men who once looked stereotypically masculine can walk into whichever public toilet they choose with no one batting an eyelid. But is a transgender person like Caitlyn Jenner merely posing as a woman or has she shapeshifted?

For Shakespeare’s audiences, watching a boy actor dressing as a girl named Viola who dresses as a boy to woo another boy dressed as a woman called Olivia was funny. As Ricky Gervais has made a tedious habit of pointing out, we cannot laugh at Caitlyn Jenner without facing a backlash. But still transition causes anxiety. One part of us finds shapeshifting creative and fascinating, another finds it transgressive and unnatural.

In this wide-ranging study, John Kachuba argues that our interest in what the American Psychiatric Association calls ‘gender dysphoria disturbance’ is only the latest manifestation of our long-established fascination with shapeshifting in all its guises. It is ubiquitous in literature and folklore, from the countless metamorphoses in Greek