Why do some languages change at a different rate to others? What causes one language to change faster than another? Why do some language families have many languages and why do some families only have a few? According to the latest version of the Ethnologue (Lewis 2013), there are 7547 languages in the world divided into at least 289 language families (including isolates).1 However, there is substantial variation in the number of languages — what I will call diversity — in each family. The Niger-Congo and Austronesian language families contain 1543 and 1255 languages respectively — about 37% of the total alone. At the other end of the scale, 160 language families have only one surviving language…