The Supreme Court justices are back from vacation. They’ve picked up their robes from the cleaners — Alito’s had a pesky mustard stain — and are reassembling Monday to hear the first of the new term’s arguments. Another term means a fresh set of cases and issues; the court will grapple with affirmative action, the death penalty, labor unions and redistricting. And, because of the incessant drumbeat of time, it also means that each justice is a year older. That might just change how they vote.

There’s an old saw, often mistakenly attributed to Winston Churchill, that goes something like this: “If you’re not a liberal when you’re 25, you have no heart. If you’re not a conservative when you’re 35, you have no brain.” A person should start left and drift right, and not the other way around, the adage suggests.

But when it comes to Supreme Court justices, growing older appears to incite a trend in the opposite ideological direction. One prominent measure of judicial ideology — the Martin-Quinn score — illustrates this tendency. These scores, as DW-Nominate does for legislators, use the justices’ votes to quantify their position on a left-right spectrum. A more negative score means a justice is further left; a more positive score means she’s further right. The scores are based on data from the Supreme Court Database and are calculated back to 1937.

In the chart below, each series of connected points represents the career of one justice. The bold lines are the results of a simple linear regression for Republican-appointed or Democrat-appointed justices. Red indicates a justice nominated by a Republican president, blue a justice tapped by a Democrat.

A typical justice nominated by a Republican president starts out at age 50 as an Antonin Scalia and retires at age 80 as an Anthony Kennedy. A justice nominated by a Democrat, however, is a lifelong Stephen Breyer.

The current nine justices haven’t been shielded from these westward winds. Justice Anthony Kennedy, who joined the court in 1988, has historically had a score solidly to the right of center, but last term he found himself slightly to the left for the first time. Justice Stephen Breyer has drifted from just left of center to solid liberal. Even Justice Antonin Scalia has liberalized somewhat from his extremely conservative positions in the late 1990s.

Why might this happen? What forces act upon a justice as he or she ages on the bench? Here are a few theories that emerged after I poked around and talked to some experts:

Martin-Quinn score co-creators Andrew Martin and Kevin Quinn, along with their co-authors, have observed a similar “ideological drift” by justices. “Drift to the right or, more often, the left is the rule, not the exception,” they wrote in a 2007 journal article.

Certain justices are historical torchbearers of this liberalizing phenomenon. Justice William Brennan was nominated to the court, as a moderate, by President Dwight D. Eisenhower in 1956. He went on to become the court’s liberal hero, and he received the Presidential Medal of Freedom from President Bill Clinton in 1993. Blackmun is another, having significantly altered his positions leftward on the death penalty, federalism and women’s rights, as Martin et al. point out.

Many on the right, including some Republican presidential candidates in the most recent debate, have expressed their dismay that Chief Justice John Roberts, too, has shifted left, citing his decisions on the Affordable Care Act. As Liptak of the Times has pointed out, however, this “buyer’s remorse” is largely unwarranted: Roberts has maintained a significantly conservative record during his decade on the bench. But even his Martin-Quinn score has drifted slightly leftward during that time.

It’s likely that the next president will get to nominate at least one Supreme Court justice. If so, that president will spend sleepless nights deliberating on the perfect nominee. The Senate will then grill that nominee in confirmation hearings. Someone will eventually be confirmed and take their seat in their big black leather chair. But they won’t be the same person for long.

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