FEW readers of The Economist (or many FReepers too), one would imagine, have seen a one-eyed dwarf with bat-like wings, pointed ears and sharpened talons. Even fewer are likely to have been sodomised by one. Many of the people of Zanzibar, however, sincerely believe in Popobawa, an incubus who supposedly rapes men who doubt his existence. Isolated sightings are reported every year. Locals say that Popobawa appears accompanied by a puff of smoke, usually on Pemba, the smaller of the spice islands that make up this semi-autonomous part of Tanzania. At times of stress, Popobawa seems to go on a...