Linguistic changes involving competition between two alternative forms are investigated with three case studies on syntactic changes from the history of English. First, evidence is produced for the hypothesis that a linguistic innovation spreads at identical rates in all environments in which it is used. The empirical basis for this claim concerns a recent change in the syntactic category of American English possessive have (c. 1800-today). Second, support is offered for the hypothesis that constant rates across different contexts can be disrupted in a predictable direction if a changing linguistic form is used in contexts that are themselves unstable. The empirical material comes from changes in verb placement and topicalization in different clause type contexts in Old and early Middle English (c. 800-1200). Third, the thesis presents a scientific methodology for the appraisal of proposed causes for linguistic changes. The methodology is illustrated by the replacement of then with when as a temporal subordinator in Middle English (c. 1100-1400).