Corley, Steigerwalt, and Ward's study of unanimity on the Supreme Court sets out to make three contributions to the substantial scholarly debate about Supreme Court decision-making. First, the authors seek to demonstrate that, “contrary to conventional wisdom, the Court reaches consensus more often than not” (3). Second, they seek to explain the high level of consensus on the Court. And third, in good social-science fashion, they look to “test empirically whether the justices reach agreement when the legal answer is simply more obvious and clear” (7). Starting with a discussion of the Roosevelt Court, the authors convincingly make the case for why the era represents the end of the norm of consensus on the Court and the start of what they describe as “the era of dissensus” (14). Having established this, subsequent chapters outline their theory and then provide analysis and discussion of consensus in voting, in opinion writing, and in deciding which cases should be accepted.

That the issue of how and why the Court reaches unanimous decisions has been largely neglected has often been noted by scholars of Court behaviour, and the authors acknowledge that they are taking up invitations from others to pursue this subject. Thus on its own terms The Puzzle of Unanimity offers a valuable study for anyone interested in the Supreme Court, addressing as it does the approximately one-third of cases decided by the Court since 1953 that have resulted in unanimous rulings. The book offers the first comprehensive study of what factors play a role (and, importantly, what factors do not) in encouraging the justices to find common ground. The study makes a clear, well-supported argument. In a field dominated by statistic-heavy studies which can leave those without the requisite maths background dazed and confused, the authors provide both the requisite statistical information and a clear, thoughtful analysis that allows even the most mathematically challenged reader to grasp the significance of the data.

The book provides a valuable contribution to the ongoing debate about Supreme Court decision-making more generally too. Among studies which seek to show why justices' attitudes, strategic interactions with other governmental actors, or legal influences are the only way in which to study Court behaviour, it is refreshing to see a study which argues that behaviour is the result of “multiple, concurrently operating influences” working in a “complex, interactive fashion” (6). While the authors openly state that they are not challenging these existing studies, and that they support the attitudinal model of Court behaviour, their study both builds on the existing research and challenges it by looking to find areas of overlap between the existing theories. For anyone with experience of this field who has perhaps found that the single-theory focus of much of the work raises more questions than answers, The Puzzle of Unanimity presents a welcome change. The authors' findings, that legal factors play a role in shaping justices' voting and writing about cases, is particularly encouraging after a long period in which attention has been focussed elsewhere. Their conclusion that “members of the Court take seriously their role as jurists through a concern for the broader institution of the Court as well as for the rule of law itself” (167) rightly takes seriously the justices' own statements about the importance of legal rules and procedures in their own decision-making processes.

If there is a criticism to be made of The Puzzle of Unanimity, it is perhaps that the overall conclusions will not come as much of a surprise to anyone with an interest in the Supreme Court. That unanimous decisions are more likely in cases where there is a high degree of legal certainty about the issues raised, where the issues raised are least controversial (so not civil rights or civil liberties cases), and where the justices believed it was important to clarify the law and issue a final ruling will seem reasonably obvious to anyone who has spent any time studying the Court. That said, it is nevertheless helpful to find that statistics reinforce this view of the Court and its behaviour. Equally, the questions asked along the way to these conclusions and the issues raised by the study overall do offer new and important insights into the work of the Court. While, as the authors acknowledge, this book is the start and not the end of the debate, it is a strong opening contribution.