In this chapter and the next, we examine a number of hypotheses relating to diffusion, the study of the progress of linguistic innovations. The hypotheses attempt to answer different questions. First, we ask who the innovators are. The answer differs with the social circumstances surrounding the innovation, as we shall see, and we shall look at several studies from urban dialectology for the light they shed on the social factors behind the pattern of diffusion. Then we look at the topic more narrowly, seeking to discover what linguistic elements are the vehicles of innovation. A promising hypothesis, known as lexical diffusion, posits that the lexical formative is the primary vehicle for phonetic change at least. Finally, in the next chapter, we ask how innovations spread geographically and develop a geolinguistic model to account for it.

Real time and apparent time

Clearly, any study of the spread of a linguistic innovation will necessarily be comparative. The data must include evidence for the same population or at least for a comparable population from at least two different points in time. Ideally, one would like to have the results of a survey designed to elicit a particular variable at a particular time and then a replication of the same survey given to the same population after a lapse of several years.