In argumentative text types, evidential markers serve to construct credibility, authority, and power. Informed by corpus-assisted discourse analysis, this paper investigates the development of adverbial, phrasal, and clausal inferential adverbials in the Corpus of US Supreme Court Opinions spanning the 1790s to the 2010s. The analysis reveals that therefore constitutes the most common adverbial form but shows a continuous decline throughout the entire time span, with consequently and hence being way less common and demonstrating only minor frequency changes. The prepositional phrases for . . . the reason(s) . . . show an interesting increase but at a lower level. The clausal constructions that is why, this is why, and that’s why represent a fairly recent and infrequent format since the 1940s, with that is why enjoying a steep increase until the 2010s. The qualitative analysis explores the collocations of therefore and for . . . the reasons . . . and examines the text components in which these formats as well as that is why/that’s why are typically found. The findings reveal patterned collocates, including verbs denoting legal agency (AFFIRM, REVERSE, DISSENT), cognitive verbs (CONCLUDE, HOLD, THINK), communicative verbs (PRAY), and the legal adverb void as well as point to striking developments in terms of the subjects which these collocates take. While therefore, for . . . the reasons . . ., and that is why serve as adverbials in the majority and minority opinions, the contracted form that’s why is only used in the transcripts of court trials. It is concluded that the court opinion continues to be an authoritative formal genre. However, there seems to be a shift to a less abstract and more transparent and accessible style. The changing polyphony of voices suggests colloquialization, increasing informality, and democratization of the genre which may be linked to a decreasing discursive authority of the majority opinion of the court.