That language contact results in linguistic change is a widespread assumption, but enjoys relatively little empirical support. In this paper we propose and test a scientific method to determine the effects that contact languages exert on each other. We illustrate its utility with preposition stranding in North American French, widely believed to result from centuries of intense contact with English. Systematic analyses provide no evidence of convergence, revealing instead that the offending construction arose from analogical extension of a native French construction to a novel context. We conclude that grammatical convergence must be proved on a case-by-case basis, using methodology capable of distinguishing internal from contact-induced change.