Justice Clarence Thomas apparently has one additional requirement. Without exception, the 84 clerks he has chosen over his two decades on the court all first trained with an appeals court judge appointed by a Republican president.
That unbroken ideological commitment is just the most extreme example of a recent and seldom examined form of political polarization on the Supreme Court. These days the more conservative justices are much more likely than were their predecessors to hire clerks who worked for judges appointed by Republicans. And the more liberal justices are more likely than in the past to hire from judges appointed by Democrats.
Each justice typically hires four clerks a year. Since Chief Justice John G. Roberts Jr. joined the court in 2005, Justice Antonin Scalia has not hired any clerks who had worked for a judge appointed by a Democratic president, and Justice Samuel A. Alito Jr. has hired only two. At the other end of the ideological spectrum, only four of Justice Ruth Bader Ginsburg's clerks on the Roberts court came from judges appointed by Republicans. The early data on President Barack Obama's two appointees, Justices Sonia Sotomayor and Elena Kagan, show a similar pattern.
By contrast, Chief Justice Warren E. Burger, a conservative appointed by President Richard M. Nixon who led the court from 1969 to 1986, hired roughly even numbers of clerks who had worked for judges appointed by Democrats and Republicans. Judge Richard A. Posner, a generally conservative judge appointed to the bench by President Ronald Reagan, clerked for Justice William J. Brennan Jr., a liberal.