JEROME FRANK, a mid-20th-century legal thinker, is said to have claimed that justice is a function of what the judge had for breakfast. Don’t let their black robes, serious miens and pledges of fealty to the law fool you, Mr Frank warned: judicial decisions are not cool applications of objective legal principles. Rather, they are manifestations of personal predilections and biases. Mr Frank’s observation seems to apply all too well to today's Supreme Court. When ruling on big, controversial cases, the justices split fairly reliably along party lines dictated by their appointing presidents. It wasn’t always this way. Until 1937, as Adam Liptak of the New York Times reported last week, party simply wasn’t a factor in high-court decisions. Only in recent decades have party politics infiltrated the marble halls of the Supreme Court, and only in the past few years have they become the best predictor of its major rulings. The Supreme Court has never divided along partisan lines as neatly as it does today.

A high court that splits into ideological camps while purporting to provide “equal justice under law” calls into question its very legitimacy. It makes a mockery of Chief Justice John Roberts’s hoary claim that a justice’s job is to “call balls and strikes and not to pitch or bat". It gives one the sense that when the Chief Justice asserts he has “no agenda,” he’s protesting a bit too much.

Indeed, new research by three political scientists shows just how avidly the justices go to bat for causes they identify with. In their paper, Lee Epstein of the University of Southern California and two colleagues examined 4.519 votes in 516 Supreme Court free-speech cases from 1953 to 2010 to determine whether “justices defend the speech they hate.” The answer: rarely. Contrary to stereotypes about the relative friendliness of conservatives or liberals to free-speech claims generally, Ms Epstein and her co-authors found that the justices are “opportunistic free speechers.” Some principle might be found to account for the suspicious patterns in their votes, but the evidence looks pretty damning. Justices’ votes “are neither reflexively pro- or anti-the first amendment”; they are, instead, for or against “the speaker’s ideological enclave.”

For example, if the speaker seeking first amendment solace is a pro-lifer rankled by restrictions on protests near abortion clinics, his rights are very likely to be recognised by Justice Clarence Thomas, a conservative, but not by Justice Ruth Bader Ginsburg, a liberal (see Hill v Colorado). And if the speaker is a high-school kid holding up a banner reading “Bong Hits 4 Jesus” on a school trip, you can expect Justice Thomas to harumph while Justice Ginsburg rises to the defend the student’s free-speech rights (see Morse v Frederick). Right-wing justices tend to uphold conservative speakers’ rights and rule against liberal litigants; liberal justices smile on their ideological friends and frown at their foes, too.

Why would judges on the highest court in the land succumb to such apparent prejudice? Ms Epstein et al have an explanation: “because judges are humans.” Everyone is subject to “in-group bias,” a well-established human tendency “to evaluate our own group or its members more favourably than outsiders,” and judges are no exception. They are fated to favour their own, no matter how strongly they claim to be neutral observers balancing the scales of justice with blindfold firmly in place.

But that explanation should satisfy no one. If the justices are to live up to their title, they should, more or less, judge blindly. The extent of a citizen’s freedom of speech should not turn on the degree of affinity between his political views and those of a majority of the justices. Church and state cases should not be decided by a vote of five conservative Catholics against three liberal Jews (joined by one liberal Catholic), as we saw in Town of Greece v Galloway last week. Nor should the contraceptive mandate for pious employers in Obamacare stand or fall depending on the religious and ideological makeup of the Supreme Court. Don’t be surprised if the Hobby Lobbyand Conestoga Wood cases are decided on apparently ideological lines when that decision comes down in the next few weeks.

It bears noting that the Supreme Court’s current liberal and conservative wings are not—not remotely—equally implicated in the shady free-speech-for-my-friends racket. As this table from the authors' comprehensive study shows, the righties on today’s court appear to be significantly guiltier of in-group bias than are their liberal colleagues:

Justices Breyer and Ginsburg appear to support liberal speakers over conservative ones, but the discrepancy is not nearly as significant as that evinced by the Court’s five conservatives. (Justices Kagan and Sotomayor have too few votes to be analysed.) For Justice Thomas, there is a 42.3% divide between his support for conservative speakers and liberals; for Justice Scalia, the figure is 44.5%. For Chief Justice Roberts, the great self-styled umpire, the gulf is 44.8%. By comparison, Justice Breyer is an impressively equal-opportunity free-speecher. He favours liberal speakers only 1.9% more often than he blesses conservatives. Justice Ginsburg’s divide is 13.2%.

This Supreme Court term provides more evidence of the current liberals' relative neutrality vis-a-vis their conservative brethren. Justice Breyer recently departed from his liberal colleagues to join the majority in a case upholding a Michigan ban on affirmative action. And during the January oral argument in National Labour Relations Board v Noel Canning, no justice in the liberal bloc seemed willing to give President Obama a free pass on his dubious recess appointments stemming from a fight with intransigent Senate Republicans in 2011. Justice Kagan suggested to Donald Verrilli, the solicitor general, that Mr Obama had injected the recess appointments clause with "a new purpose that nobody ever intended it to have". That indictment, coming from an Obama appointee, represented a notable counterpoint to the Court's trend towards increasing partisanship. There are few signs the conservatives intend to reciprocate.