Observation of language change caused by language contact tend not to support theories put forth by Meillet, Jakobson, Weinreich, and others that there are linguistic prerequisites for such change. A theory of the social factors that determine what kinds of language change will take place as a result of contact begins with the recognition of two different types of linguistic interference: borrowing, or the incorporation of foreign features into a language; and substratum interference, which occurs when speakers shifting to a target language fail to learn the language perfectly. In borrowing situations, the most important variable for predicting the degree and type of interference is the relative intensity of cultural pressure from source-language speakers on the borrowing-language speakers. In language shift changes, intensity of cultural pressure is still vital, but changes enter the language as soon as the shift begins and do not require a period of time to develop. Markedness is valuable in predicting which features of the target language are least likely to be learned, with universally marked features predictably the last to be learned. (JB)