But such desires didn’t ‘develop’, or didn’t in David. He could remember waking up with them, as one does with the symptoms of disease, headache, sore throat, fever, and it seemed to him they crippled him into his adolescence much as polio might have. He could still sometimes feel the vestigial limp.

Until they went to school, David and Diana were two halves of an harmonious nature. Oh, they quarrelled occasionally, but their griefs were far more often mutual and sympathetic, a wonderful defence against their instinctively gentle mother, whose discipline was guilty and sporadic and easily deflected by their indignant howling.

Acutely aware that they lacked a father, their mother was particularly careful to supply David with the masculine toys he might have chosen (or might not), soldiers, pop guns, cowboy hats, a doctor’s kit. But he and Diana played with all their toys interchangeably. If David was the father and Diana the mother for her dolls, David bathed and changed and pretended to read to them in imitation of the only parent they had. They also spent hours designing battle fields for the soldiers, digging trenches, establishing look-out posts, arranging the troops in companies and scouting parties. At the end of the day, there was never a battle. They might occasionally pelt the soldiers with ripe plums from the overburdened tree in the garden, but it was more like breaking up building blocks for the next project than an act of destructive violence.

Out in the world, they held hands, David to protect Diana from cars, she to protect him from dogs. In bed at night, they sang through their crib slats to each other, giggled and were suddenly both deeply asleep when their mother came in to silence them.

It never occurred to either of them to tattle on the other. Their criminalities were shared. Even in the attention twins always attract, they didn’t compete but closed ranks to be sure they weren’t fussed over too intimately or deprived of treats adequate for them both.

‘Thank you, and one for D,’ each learned to say. They had the same name for each other, which even their mother couldn’t use since it didn’t distinguish them from each other. For themselves they didn’t need to distinguish. Differences didn’t matter.

Yet in a single day David learned not only to drop Diana’s hand but to call her all the new names of the kindergarten playground, chief of which was ‘girl’, the generic insult from which all others stemmed, Diana was a girl among the other girls, the untouchables.

Some of the other boys were also cursed with sisters, but they were older or younger, not humiliatingly in the same class. Diana expected to sit next to David, to roll out her nap rug next to his, to share juice and cookies.