Dunbar’s number is 150: roughly how many people one person can keep up a real relationship with. Robin Dunbar got it in the early 1990s by plotting the size of primates’ neocortex against the size of their groups and reading off where a human brain would land, which came to about 148. He and others have since pointed to it in village sizes, army companies and Christmas card lists, and it gets quoted as if it were a constant. In 2021 three researchers in Stockholm ran the primate data again with newer methods. Their best estimates of the average came out anywhere from 16 to 109, depending on the method, and the 95 per cent ranges ran from 2 to 520. The same data, in other words, will support a life with two people in it or one with five hundred. I don’t know which side has it right. What I notice is how much of the number’s career came from its being a single number. Nobody was ever going to write a book called Somewhere Between Two and Five Hundred.