Robin Dunbar, a British anthropologist of renown, has argued that the number of people with whom one can maintain stable social relationships is around one-hundred and fifty. This is now known as Dunbar’s number. He suggested that the limit is imposed by the brain’s processing capacity. Evidence from prehistory shows that the size of stable social groups varied from about one-hundred to two-hundred and fifty, but Dunbar’s number is still a useful idea linking brain size and social group size. Why am I going on and on about all this? This morning I found an electronic-mail message from my No. 1 son, who forwarded me something from a blog that mentions Dunbar’s number. In the blog, the number is used to show why people listen to prophets of doom, who are often closer than their own relatives, but the argument is hopelessly garbled. If people’s capacity to follow prophets is limited by their brain size, prophets of boon would have an equal chance as the prophets of doom. In my quick response to my son, I mentioned that Dunbar’s number is often on my mind in connection with our eventual return to prehistory. That is, posthistory. Once again, we will be with as many people as our brains can process in sufficient detail. This I am quite sure about. I am only confused about my status: am I a prophet of boon or doom?