When I found myself unexpectedly pregnant with twins more than thirty years ago, my doctor described identical twins, or monozygotes, as ‘a freak of nature which can happen to anyone’. Fraternal twins, by contrast, tend to run in families, or be the result of fertility treatment or the mother being older than I then was. So I prepared for monozygotes. The babies’ father, whom I didn’t know very well, said, ‘We can’t possibly have two babies looking the same; let’s decide now that we will treat one as a human and the other as a dog.’ I pushed back on that, but I did look forward to years of experiments, high jinks and easy money as I imagined hiring out my armful of gold dust to social scientists. No such luck! My sons emerged into the world different in everything, from colourways to character. I changed tack at once, throwing away my plans and becoming impatient with anyone slow to understand that fraternal twins were no different from ordinary brothers, except that people compared them even more than siblings are usually compared and that this was likely to be a bore for them.

William Viney, although he himself hails from the monozygote aristocracy, takes a less binary approach in his cultural history of twins. He points out that the biological distinction between identical and fraternal twins – the first being the result of a fertilised egg splitting, the second the result of