Just how conservative is the Supreme Court, anyway? It's a question that has dogged constitutional scholars for years, as they've tried to parse the opaque language and muddled writings of judges moving through the confirmation process. Today's court, headed by John Roberts with seven justices appointed by Republican presidents, is generally considered more conservative than the Supreme Court of the 1950s, for example, when Earl Warren oversaw its unanimous decision in Brown v. Board of Education. But it's hard to compare the current court—and today's justices—with, say, the Burger court of the 1970s, which, with six Republican-appointed justices, decided Roe v. Wade.

John McCain, for one, doesn't seem to want to take any chances. Last week, he joined a long line of Republican presidential candidates who have pushed for a more conservative court when he promised to make Samuel Alito and John Roberts his "models" for judicial appointments.

But how conservative would a McCain presidency make the court—and how conservative is it already? The answers to these questions may be found in a new paper by Richard Posner, a judge who sits on the Seventh Circuit Court of Appeals, and William Landes, a law professor at the University of Chicago, that is now making its way through the academic community. In "Rational Judicial Behavior: A Statistical Study," Posner and Landes use a database that includes the political background and voting records of the past 70 years of Supreme Court justices—who appointed each justice and how the justices decided every case—to come up with a ranking, from most conservative to least conservative, of the 43 justices who have served on the court since 1937.

Their conclusion: Four of the five most conservative justices to serve on the Supreme Court since Franklin Roosevelt, including Roberts and Alito, are currently sitting on the bench today. Justice Anthony Kennedy, another current Republican appointee, is ranked No. 10. (The table has a full list.) Justices Ruth Bader Ginsburg and Stephen Breyer, the two current justices nominated by Democratic presidents, are among the 15 "least conservative" justices of the past 70 years. Thurgood Marshall, who became the first black Supreme Court justice when he was appointed in 1967, has the most liberal voting record on the list. Clarence Thomas, the second black justice, who was appointed to the court in 1991, is ranked the most conservative.

NUMBER OF CONSERVATIVE VOTES ON U.S. SUPREME COURT, 1937-2006

10 "Most Conservative" Justices Justice Name Percentage

Conservative

Votes Thomas .822 Rehnquist .815 Scalia .757 Roberts .753 Alito .740 Burger .735 O'Connor .680 Powell .677 Whittaker .673 Kennedy .647

10 "Least Conservative" Justices Justice Name Percentage

Conservative

Votes Marshall .211 Douglas .213 Murphy .241 Rutledge .247 Goldberg .248 Brennan .265 Black .283 Warren .308 Ginsburg .312 Cardozo .333

These findings may not come as a surprise to political scientists, who have devised a range of techniques over the years for calculating judicial nominees' political ideologies, often based on their records before joining the Supreme Court. But Posner and Landes have taken their research a step further, examining the actual voting records of justices while they were sitting on the bench. To do this, they've relied on a database created by Harold Spaeth, a political scientist at Michigan State University, which codes each vote made on the Supreme Court between 1937 and 2006 as either "liberal," "conservative," "mixed," or "both." If a justice votes in favor of a defendant on a criminal procedure case, that vote is considered "liberal." If the justice votes against the plaintiff in a civil rights case, the vote is considered "conservative."

Some of the results of the authors' analysis are familiar, if not reassuring. The study demonstrates, for example, that Supreme Court justices, in spite of their reputation for impartiality, really do seem to vote along ideological lines. With only a few exceptions, for 70 years, Republican-appointed justices have tended to vote conservatively, and justices appointed by Democrats have tended to vote liberally. "A lot of this just confirms what everybody already knows," says Landes.

But the paper reveals more than that. For one thing, it demonstrates just how much every vote on the Supreme Court counts—particularly, it seems, when the court is leaning conservative. The authors were surprised to find a dramatic pooling effect over the years every time a conservative justice joined the court. "The larger the fraction of justices appointed by Republican presidents," they write, "the more conservatively each Justice [votes]." With McCain promising to nominate conservatives, this finding appears to have real significance today: The more conservatives join a conservative court, the more conservative each justice gets.

The same thing doesn't appear to be true, curiously, for left-leaning judges. The authors find that the court's liberal justices are driven to vote more ideologically not when their numbers grow but when they begin to drop. The fewer justices there are on the court appointed by Democratic presidents, in other words—meaning the more outnumbered the liberal justices are—the more liberal those justices get. Even when the majority shifts only a small amount, from 5-4 liberal to 5-4 conservative, liberal justices tend to vote more liberally about 3 percent of the time. "There's a real polarization effect," says Landes.

Today's court, it seems, appears to be in just this situation, with increasingly ideological justices moving toward the ends of the political spectrum. But there may be a solution already in the making: The political atmosphere surrounding judicial nominations, the authors point out, has a dramatic effect on not only what kinds of judges are nominated but how they vote once they're on the bench. The more Republicans there are in the Senate, according to the study, the more conservatively the justices they appoint tend to vote. But over the years, when the number of GOP senators drops—something many political experts think may be likely this fall—so, too, does the career conservative rating of the justices they appoint to the bench. "It's an interesting finding," says Landes. "There's some real predictive power here. You can determine a lot about how each justice is going to vote based on the political party of the president who appointed them and the composition of the Senate."