The big question about John G. Roberts is whether he is too political, or, to use the popular phrase, too "ideological". The White House is counting on the new Supreme Court nominee's reputation as unpolitical to win him confirmation. Yet opponents are already reporting that Judge Roberts once co-wrote a brief from the solicitor-general's office advising that Roe v Wade, the case legalising abortion, "should be overruled". Reporters are studying every detail of his life, down to the fact that his middle name is Glover. And the hearings have not even started.

The premise here is that blander is better. Knowing more about the candidate's past ensures that his performance as a justice satisfies the country later. The most common example is the nasty rejection of conservative nominee Robert Bork in the 1980s. Blocking Senators were jubilant, believing that they had managed to flush Judge Bork out as an ideologue.

But these assumptions are questionable. A bland nominee does not always make for a bland justice. And a nominee's background cannot always tell us how he will tilt once upon the bench. Consider the current Court, starting with Justice David Souter. Few conservatives opposed the nomination of the Weare, New Hampshire man to the Supreme Court back in 1990. He seemed bland. What is more, New Hampshire is a conservative state - "Live Free or Die" is its motto. Judge Souter appeared to be a typical New Hampshirite. Some conservatives even nursed the hope that his ascent to the bench might create a majority to overturn Roe v Wade.

Instead it was the very same David Souter who, as a justice, supplied the vote that gave Roe defenders a majority in a subsequent test case, Planned Parenthood v Casey. The conservatives were outraged. Just recently, Justice Souter infuriated them yet again when he sided with property developers - and against property owners - in a case that asked whether the government may take land from owners on behalf of developers if the act benefited the community. One citizen was so angry that he launched proceedings in tiny Weare to have Justice Souter's two-story farmhouse confiscated - on grounds that the property might better serve the community if developed as a hotel.

Then there is William Rehnquist, the ailing chief justice. At the time of his nomination commentators reported every detail of the Rehnquist life and attitudes right down to his middle name - Hubbs. They noted with excitement that Mr Rehnquist had made a speech declaring that it would be "not at all unreasonable" for the White House to ask the Supreme Court to reconsider Miranda v Arizona, a decision upholding a citizen's right to silence upon arrest. Recently, Justice Rehnquist led the court in backing the constitutionality of the Miranda process.

Going farther back there was Republic appointee Harry Blackmun, who wrote the opinion that became Democrats' all-time favourite, Roe v Wade. Then there was Earl Warren, aka Eisenhower's Greatest Regret. President Eisenhower and his advisers viewed their fellow Republican, a California governor, as moderate and therefore acceptable. Yet the Warren Court proved radical - flamboyantly expanding the federal government's role in education. Earlier still, President Franklin Roosevelt knew and loved Justice Louis Brandeis, whom he nicknamed "Old Isaiah". The president told himself that he could count on Brandeis to support the New Deal's biggest institution, the National Recovery Administration. Yet Brandeis joined the rest of the justices in determining the NRA's very existence was unconstitutional.

Why do justices prove so unpredictable? Obviously, people change. Nominators often delude themselves about nominees (Eisenhower did about Warren). Expectations also change. When Justice Rehnquist arrived at the court, the country had trouble imagining a more conservative justice. But then Antonin Scalia and Clarence Thomas arrived. Roosevelt moved left, making the court look rightwing.

But the greatest factor nowadays is the intensity of the selection process. Modern White Houses have been seeking out candidates with obscure views for so long now that ambitious jurists have spent entire careers establishing reputations of unplaceabililty. If obscurity is the price, then lawyers will pay it to win their profession's greatest prize. Thirty-two is an impressionable age. It is the age of John Roberts when he watched the assault on Judge Bork. It is not hard to imagine why Mr Roberts later avoided controversy.

The general wisdom is that this trend is good because it emphasises professional skill. But people who spend their lives proving themselves to be unknowable often do not know themselves very well. Yet once ensconced at the Supreme Court, they must take positions. As a result, they may be less predictable, not more.

The reply to all this from Republicans is to blame Democrats: things have to be this way, otherwise Democrats will block all their candidates. Democrats argue the reverse. But we can still acknowledge the situation. If the best thing about Judge Roberts' reputation is that he has none, that itself is a problem.

© Copyright 2005 Financial Times