This article presents a corpus-based study of relative clauses involving the head noun reason and the competing relativizers why, that, and zero. The analyses focus on specific constellations of antecedent expressions (e.g., the reason) and subject types in the relative clause (e.g., personal pronouns). In an extensive newspaper database, American English is found to display with every single combination a lower aggregate proportion of the formally marked relativizers than British English. Additional historical explorations leave no doubt that explicit relative clause marking has lost ground over the last few centuries and that American English has been leading this development for at least 150 years. In both British and American English, and in line with the Complexity Principle, the complexity contrasts associated with different choices of antecedent expressions and relative clause subject types are shown to be matched by corresponding contrasts in terms of grammatical explicitness, with the most complex environments favoring why or that and the least complex leaning toward the zero variant.