What explains the low number of 5-4 cases so far this Term? I have a pet theory — really just idle speculation at this point — that Chief Justice Roberts may have found a sweet spot aiming towards the middle by relying more heavily on the Marks rule than his predecessor Chief Justice Rehnquist.

  First, a bit of background. It takes five votes for an opinion to become a binding "Opinion of the Court." When there is no one majority opinion, the Court has generally embraced what is known as the Marks rule, after Marks v. United States, 430 U.S. 188 (1977). The Marks rule is (essentially) that if there is no one majority opinion for the Court, the controlling opinion for future cases is the narrowest decision in favor of the winning side. As the Court put it in Marks, "When a fragmented Court decides a case and no single rationale explaining the result enjoys the assent of five Justices, the holding of the Court may be viewed as that position taken by those Members who concurred in the judgment on the narrowest grounds."

  There isn't any one "opinion of the Court" in such cases, but the lower courts must follow the narrowest opinion for the winning side as if it were the majority opinion. And in recent years, the Justices themselves have largely treated the controlling opinions under Marks as if they were majority opinions for purposes of stare decisis. An example is Coker v. Georgia, where the binding opinion under the Marks rule was Justice White's plurality decision; in the recent litigation in Kennedy v. Louisiana, that plurality opinion was treated essentially as if it were a majority opinion.

  This is just a guess, but I would speculate that the low number of 5-4 cases may result in part from Chief Justice Roberts being more willing than the late Chief Justice Rehnquist to write or assign controlling plurality opinions instead of trying to force majority opinions out of a closely divided Court. In particular, the current configuration on the Court lets Chief Justice Roberts aim for a center-right position in which he starts with Alito, Kennedy, and himself on board. By taking a center-right position, he has a chance of picking up a vote from one of the liberal-leaning Justices who feel they can gain more by joining a center-right opinion than by dissenting. End result: fewer 5-4 decisions.

  The obvious risk here is that a center-right approach will alienate Justices Scalia and Thomas, who might feel that they can't sign on to the resulting opinion. This has happened in a few major cases this Term: see, for example, Baze v. Rees, the recent death penalty case, or Crawford v. Marion County Election Board, the recent voter ID case. Both were 6-3, but both featured a controlling center/center-right opinion and a separate Scalia/Thomas opinion (one of which added Alito). Thus, a counterintuitive by-product of having fewer 5-4 decisions is that there may also be fewer majority opinions.

  But losing a majority opinion isn't really a problem as long as the courts consistently follow the Marks rule. If Thomas and Scalia break off and write a concurring opinion that would draw the line away from the center, then under Marks the center-right plurality opinion is still the controlling opinion going forward. The center-right opinion that Roberts either authored or assigned remains the controlling opinion even if it is not an "Opinion of the Court" with a majority on board.

  Now you're wondering, why would Roberts do that? Well, first of all, recognize that this is pretty much what Roberts said he would do in his interview with my colleague Jeffrey Rosen in the January 2007 Atlantic Monthly. I know, I know: we're all way too cynical to actually believe what a person says to the press. But I think Rosen's interview is worth a re-read:

Promoting unanimity will not be an easy task, Roberts acknowledged, after years of "the personalization of judicial politics." He said that he had to emphasize the benefits of unanimity for individual justices, in order to influence what he called the "team dynamic." "You do have to [help people] appreciate, from their own point of view, having the Court acquire more legitimacy, credibility; [show them] that they will benefit, from the shared commitment to unanimity, in a way that they wouldn't otherwise," he said. Roberts added that in some ways he considered his situation—overseeing a Court that is evenly divided on important issues—to be ideal. "You do need some fluidity in the middle, [if you are going] to develop a commitment to a different way of deciding things." In other words, on a divided Court where neither camp can be confident that it will win in the most controversial cases, both sides have an incentive to work toward unanimity, to achieve a kind of bilateral disarmament.

  More broadly, it seems to me that this strategy would be a pretty savvy approach for a Chief Justice on a divided Court who is deeply concerned with the stability and predictability of the law. Why? Aiming for the center-right secures the center, and securing the center makes the center-right opinion the consistently controlling opinion under Marks. In contrast, trying to get all five conservative-ish Justices on board a single opinion risks unpredictability if the five don't hold. Under the Marks rule, if the center vote breaks off, the holding of the Court goes wherever that center vote goes. That's likely to lead to less predictable decisionmaking; better to to try and secure the center first. Indeed, according to SCOTUSblog's latest posted STATPACK, as of May 1, Chief Justice Roberts and Justice Kennedy have joined the same opinion 88% of the time this Term — the second most common pairing of Justices, just behind Souter/Breyer at 90%.

  Anyway, that's my tentative speculation about why there are fewer 5-4s so far this Term. I may be way off, and at the end of the Term I may be singing a very different tune, but that's my guess for now.