I’m not afraid to admit it, nowadays: I’m an only child. Back when I was a child, in the 80s, it was a sort of novelty, us singular children– most of my friends had at least one sibling, and some had two or three more. It wasn’t until I was 10 that I even met another only child.

I didn’t mind being an only child, and it was only romanticized notions of siblinghood I longed for– the fantasies I had of a sister always revolved around the idea that she was close in age to me, and had my exact int...