The Supreme Court justices farm their law clerks from the Circuit Courts. That is, a law student from an exceptional law school with stellar credentials first lands an appointment to one of the Circuit Justices. He or she is selected however based on another consideration: his politics. Oh, it is never so direct, I suppose, but viewpoints and attitudes are culled from careful screening and interviewing. The justices, who so disclaim political considerations during their own "confirmation hearings" before the Senate Judiciary Committee, are looking for clerks of like minded "liberal" or "conservative" bias. How do they find them?
For Supreme Court justices, the method is to rely upon the recommendation of a trusted "liberal" or "conservative" Circuit Court justice who has had the opportunity to observe and evaluate the political preferences of his or her clerk over the last year or so. The pattern follows the motive: conservative Ninth Circuit justices recently recommended apparently conservative minded law clerks for plum appointments to their corresponding conservative Supreme Court justices. The liberal members of the Ninth Circuit Court of Appeals recently did the same. It is likely a pattern for all the Circuits that operate as "farm systems" from which 39 clerks are selected each year for the court: 4 for each Justice, and one each for 3 retired justices.
One of the evidences of "group think" is to filter out people who think differently than you. I suspect the U.S. Supreme Court, like any organization, is vulnerable to the "all too human" practice.
While there are a few exceptions, Justices of the Supreme Court mirror the politics of their appointing Presidents. Social justice issues often produce split decisions reflecting those political biases. Essentially, there is a "conservative" and a "liberal" block of appointees on the bench, and the split currently is often 5-4 for the conservatives.
There is nothing nefarious in this whole process, but I still find it disturbing. What could be more boring than to have a bright mind reflect back to you your usual political outlook on a social justice issue such as access to abortion, the holding of a religious demonstration in proximity to a serviceman's funeral, or the presence of a religious symbol on public property? How exciting can it be to listen to your clerk exalt the virtues of free enterprise and employer discretion in deciding whether to grant class action status to over a million women seeking gender discrimination damages against Wal-Mart? I would think disagreement and sharpened differences on the issues would generate more insights and better arguments, and quite possibly more balanced decisions.
When Justices of the Supreme Court state their respective positions in the pre-opinion phase, they usually do so by dry exchange of draft opinions that become the basis of argument and counter-argument, until something is hammered out as a "majority opinion" authored by one of the majority members. In contrast to this sterile movement of paper from chamber to chamber, a Justice's lively dialogue with his or her law clerk is the best opportunity for a Justice to examine the case from multiple views. Sadly, to hear an echo of one's own thought from an ideological twin produces little that is new or innovative. What harm can come from a conservative jurist engaging in a clash of views with a liberal clerk? Is either so weak minded as to be unduly influenced by the other? On the other hand, the harm is obvious when an ill-considered viewpoint goes unchallenged, or other outcomes of a decision are not vigorously explored.
In summary, the current law clerk selection process produces a "tribal" comfort but not the rich intellectual exchange that would likely produce better decisions.