Do U.S. Supreme Court justices, as some historical evidence suggests, time their retirements with an eye to the partisan or political affiliation of the president or the Senate majority? No, when it comes to retirement decisions, justices care more about power than party and policy, according to a new empirical study.

As Court-watchers ramp up speculation about possible retirements this term because of the election of President Barack Obama -- viewed by some as a "favorable" political climate particularly for potential retirees justices John Paul Stevens, Ruth Bader Ginsburg and David H. Souter -- two political scientists at Santa Clara University found that justices commonly are not "strategically" retiring in the modern era despite historical evidence that some justices have. Instead, their retirement decisions are influenced by three dominant factors:

• Their sense of ideological mission.

• Their institutional influence and leverage when writing majority opinions.

• The time of next presidential election.

"Specifically, Justices who are more ideologically distant from the Court median are less likely to retire, as are Justices who are writing more majority opinions and thus more engaged and influential in determining the content of the Court's doctrines," concluded professors Terri Peretti and Alan Rozzi in their study, "Modern Departures from the U.S. Supreme Court: Party, Pensions, or Power." The authors examined Supreme Court retirements from the 1953 through the 2006 terms.

"We find that justices are less inclined to leave the bench when fulfilling an ideological mission by "fighting the good fight" from the wings or when steering the Court by writing majority opinions that shape legal policy," they concluded.

Justices are more likely to retire the more distant the next presidential election, they added. But although the timing of the next presidential election might be considered a political calculation, they wrote, it is not actually a partisan calculation.

Justices, they found, do not specifically retire earlier in the terms of presidents who may share their political or partisan bent. "In fact, taking election proximity into account may be an apolitical act by Justices designed to reduce the political conflict surrounding the selection and confirmation of their successors," the authors said.

Although their evidence indicates that Supreme Court justices are not retiring strategically, Peretti and Rozzi noted that it does not prove that they neither desire nor try to do so.

"Timing their retirement to insure an ideologically-suitable replacement may in fact be a goal of Supreme Court Justices," they said. "However, that goal appears to be secondary to other objectives such as continuing to exercise power and preserving their ideological and leadership roles on the Court. Additionally, Justices may try to retire strategically but fail."

For example, they explained, unexpected health problems or an untimely death can spoil an attempt to delay retirement until political conditions are more favorable. And, more importantly, they cannot control whether a like-minded successor will be appointed.

A quick look at the historical record, the authors found, indicates that, in the roughly five decades from 1954 to 2006, 11 of the 22 justices leaving the bench (50 percent) did so under a co-partisan president, and 13 of 22 (59 percent) were replaced by co-partisan successors.

"If Supreme Court justices wish to provide co-partisan presidents with the opportunity to replace them with co-partisan successors, they are failing nearly half of the time," they said.