Language shift can take place rapidly, over a generation or two, or it can take place gradually, but continuously, over several generations. Linguists are becoming increasingly alarmed at the rate at which languages are going out of use. Overviews of the study of language endangerment usually start with a list of statistics about the number of languages in the world, the proportion considered endangered, and so on. Politics also plays an important part in language differentiation. Following nineteenth-century philosophers such as Herder, language has been considered a crucial element of national identity, with 'one state, one people, one language' being seen as the ideal. But languages do not necessarily follow political boundaries. The causes of language endangerment can be divided into four main categories such as: natural catastrophes, famine, disease; war and genocide; overt repression; and cultural/political/economic dominance.