What are the external causes of linguistic changes?

What are the external causes of linguistic changes?
【正确答案】：External causes of linguistic changes are the contacts between the speakers of different languages: the sort that occurs when a language is imposed on a people by conquest or political or cultural domination, or when cultural and other factors produce a high degree of bilingualism between adjacent speech areas. The significant influence of Norman French on the English language from the eleventh century AD supports this proposition. It can be said that any dramatic social change caused by wars, invasions and other upheavals can possibly bring about correspondent changes in language.