"It's because they are all conservatives," said Michigan State University law professor Harold Spaeth. "The court is going in a decidedly conservative direction and has been since Rehnquist became chief justice."
Since Rehnquist was sworn in as chief justice in 1986, experts say, a conservative majority has decided some of the nation's most-debated matters. Liberal issues such as affirmative action, abortion and civil rights often received conservative judgments, resulting in allegations that the court is run by politics, not legal principle.
Paul Rothstein, a law professor at Georgetown University, said the statistics aren't surprising.
"What you have is a Supreme Court that does tip on the conservative side because the justices that are sitting were appointed by conservative administrations," said Rothstein, former special council to the House and Senate Judiciary Committees. "You got Thomas and Scalia, then Rehnquist, Kennedy, then O'Connor, the least conservative of the group, but she still sits right of center."
Despite what appears to be a severe split in ideology among the justices, the high court doesn't always hand down cookie-cutter decisions and ideological lines are often blurred. First Amendment cases are a classic example, said Eugene Volokh, a law professor at the University of California, Los Angeles.
Kennedy, appointed by former President Ronald Reagan in 1988, has been the court's biggest champion of free speech, said Volokh, adding the justice is considered a moderate but often votes with the court's conservative core.
Thomas, one of the court's most conservative justices who was appointed almost a decade ago by former President George Bush, comes in second in regard to free speech, said Volokh, a former clerk for O'Connor.
In United States v. Playboy Entertainment Group Inc., the adult entertainment group challenged a law prohibiting "sexually oriented programming" during prime-time and daytime television. Programming either had to be scrambled or televised during "safe-harbor" hours between 10 p.m. and 6 a.m.
In a 5-4 decision delivered last May, Kennedy and Thomas, along with their liberal colleagues John Paul Stevens, Ruth Bader Ginsberg and David Souter, declared that the law restricts the First Amendment's free speech guarantee.
"If a statute regulates free speech based on its content, it must be narrowly tailored to promote a compelling government interest," wrote Kennedy in the case's opinion. "If a less restrictive alternative would serve the government's purpose, the legislature must use that alternative."
Justice Stephen Breyer, appointed by former President Bill Clinton in 1994, the last judge to make his way to the bench, wrote the dissenting opinion, calling the law a "realistic" alternative.
Search and seizure cases involving the Fourth Amendment also result in an unlikely ideological combination in the court, legal scholars say.
Kyllo v. United States was the perfect example of the court's two most conservative justices forging an alliance with some of the liberal justices, Volokh said.
In that case, the court questioned whether Danny Kyllo's Fourth Amendment rights were violated after a National Guard officer used a thermal imaging device to detect whether he was growing marijuana in his home.
Scalia, who authored the majority opinion, backed by Thomas, Souter, Ginsberg and Breyer, ruled against the police and affirmed that Kyllo's rights had been violated, Volokh said.
"In all these types of cases, ideology drives the justices' decisions," he said.
According to Rothstein, an existing political divide in the court has affected some the nation's most controversial issues.
"It has slowed down aggressive affirmative action in the area of race an emphasis on the rights of criminal defendants," Rothstein said.
For example, the court ruled that voting districts cannot be drawn solely to give minorities representation and certain government programs violated the rights of the "majority" because they favored hiring minorities, Rothstein said.
Jeffrey Segal, a law professor at the State University of New York at Stony Brook and co-author of a book outlining the court's voting statistics, said the conservative five often stick together in federalism cases, generally ruling in favor of states' rights.
"The divisions in the court are very real," Segal said. "There is a real strong conservative block. Sometimes O'Connor and Kennedy will split and go with the liberals, but with important issues they usually stick together."