While there is a tendency to label judges “liberal” or “conservative” — and the labels may fit, even if somewhat loosely — the real puzzle are judges who start out one way and move the other way over time.

In the population at large, and even among the intelligentsia, the usual movement over the years has been from left to right. The phrase “radical at twenty and conservative at forty” has been true enough, often enough, to become a cliche.

Most of the leading conservative intellectuals were at least liberal, and often radical, in their youth. That includes Milton Friedman, Friedrich Hayek and the whole neo-conservative movement. In politics, the leading conservative figure of the 20th century — Ronald Reagan — was a liberal in his early years.

On the Supreme Court of the United States, however, the movement has been in the opposite direction.

In an outstanding recently published book titled “Supreme Conflict,” author Jan Crawford Greenburg traces systematically the leftward movement of Supreme Court justices who were initially part of the conservative wing of that court.

Justice Harry Blackmun began his career on the High Court by voting with his fellow Minnesotan, conservative Chief Justice Warren Burger, so consistently that the media called them the “Minnesota Twins.”

Over the years, however, Blackmun moved steadily leftward and established as his judicial legacy the decision in Roe v. Wade that created a “constitutional right” to abortion out of thin air.

Justice Anthony Kennedy likewise began his tenure on the Supreme Court by voting “with Scalia and Rehnquist more than with any other justice,” as noted in “Supreme Conflict.” The liberal media savaged him as an enemy of civil rights.

Years ago, a judge who had served with Anthony Kennedy, when both of them were judges in California, warned at a social gathering that Kennedy “is not a strong person.”

Others warned against Kennedy in Washington, as detailed in “Supreme Conflict,” but the Reagan administration went ahead and nominated him anyway. Justice Kennedy’s record on the Supreme Court fully justified all these misgivings.

In the face of withering criticism, Kennedy began to move to the left — not as far left as Blackmun but far enough for some of his later decisions to contradict some of his earlier decisions. He was now lauded in the media as a “centrist,” like Sandra Day O’Connor.

Justice O’Connor also began her career voting with the High Court’s most conservative member at that time — William Rehnquist — more than four-fifths of the time. But she too moved leftward over the years, often providing the fifth vote needed by the court’s liberal justices to prevail. She too was now lauded in the media.

Although Supreme Court justices have lifetime tenure, precisely in order to give them independence, nothing can give anyone the backbone and character to stand up to criticism or to resist the blandishments of flattery and lionizing.

All the pressures are to move to the left, in accordance with the views of the liberal media and the liberal professors who dominate the law schools.

Judges who stick to the Constitution as it was written and resist the pressures to enact the agenda of the left from the bench will be depicted as narrow, dull, perhaps even stupid or morally lacking. But those who drift with the leftward tide can count on being portrayed as compassionate, brilliant or even profound.

Does this matter to federal judges with lifetime tenure? One such judge, Circuit Court Judge Laurence Silberman, has said flatly, from what he has seen, that it does.

Perhaps the most influential journalist who denigrates conservative judges and lionizes those on the left is New York Times legal reporter Linda Greenhouse.

The susceptibility of judges to such journalistic influence in general was dubbed “the Greenhouse effect,” for Linda Greenhouse, in this column 15 years ago but Jan Crawford Greenburg attributes it to Judge Silberman.

He is the one who deserves credit for identifying this judicial weakness, which is more important than coining the phrase.