Who decides which Justice authors a given majority opinion? There is, of course, the necessity that this Justice is part of the majority coalition. Beyond that though there are two main rules. The first is that the Chief Justice assigns the opinion if the Chief is a member of the majority. If the Chief is not in the majority, the second rule is that the opinion is assigned by the most senior member of the majority coalition. A Justice can assign opinion to him or herself or to any other Justice in the majority.
Five Justices have assigned majority opinions under Chief Justice Roberts. Obviously Chief Justice Roberts leads with many more assignments than all other Justices combined. Three other Justices: Scalia, Stevens and Kennedy each assigned more than ten opinions apiece. Justice Thomas has the fewest of the assigners with four. The figure below shows the majority opinion assignments counts under Chief Justice Roberts.
An interesting quirk about Justice Thomas’ assignments has to do with how he has gotten hold of this role as assigner. The Justices junior to Justice Thomas are Justices Ginsburg, Breyer, Roberts (who doesn’t count in this ordering), Alito, Sotomayor, and Kagan. For Justice Thomas to assign an opinion, he must have a coalition that is composed at least in part of liberal Justices. This likely adds to why Justice Thomas has assigned so few majority opinions.
The four cases where Justice Thomas assigned the opinion are Alleyne v. United States (2012), CSX Transportation v. McBride (2010), Walker v. Texas Div. Sons of Confederate Veterans (2014), and Dolan v. United States (2009). Justice Thomas assigned those opinions to himself, Justice Ginsburg, and the final two to Justice Breyer.
The members of the majority in those four cases were, along with Justice Thomas, Justice Breyer, Sotomayor, Kagan, and Ginsburg in all cases but Dolan and Justice Ginsburg, Breyer, Alito, and Sotomayor in Dolan.
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Pre-OT 2015 data gathered with the aid of the United States Supreme Court Database