In our podcast last week, we tried to explain why Democratic-appointed Supreme Court Justices march in lockstep in the big, closely divided Supreme Court cases, while one Republican-appointed Justice (Anthony Kennedy) cannot be counted on at all to vote with his more reliably conservative brethren and a second (John Roberts) has parted company in two of most important cases decided in his tenure.

I offered one possible explanation. Liberalism, I claimed, is a political movement with a voracious appetite for power. Conservatism is a part political movement and part intellectual phenomenon, and the intellectual component is quite diverse. There are libertarians, social conservatives, neoconservatives, paleoconservatives, and even something called crunchy conservatives. These differences are bound to be reflected to some degree in the attitudes and judging philosophies of conservative jurists.

Now, I want to offer a second, and perhaps more powerful, explanation: age. If Supreme Court Justices were appointed directly out of law school, I strongly suspect that liberals would be complaining about Democratic-appointees growing in office. It’s commonly acknowledged that the trajectory for young men is to move to the right as they begin to assume the responsibilities of adulthood, including paying mortgages and helping to support and raise children.

But Supreme Court Justices are almost always past age 50 when they are appointed. By then, the children are, or soon will be, raised; the mortgage has, or soon will be, paid off; and the Justices are looking forward to grandchildren.

These developments shouldn’t drive anyone to the left, but I believe the aging process itself often does. Why? Because conservatism, especially conservative judging, is predicated on the absence of a certain kind of sentimentality (I say “certain kind” because there is a sense in which the main strand of conservatism is quite sentimental). It is predicated on not letting “feelings” dominate the decision-making process.

The same-sex marriage opinions illustrate the point. Justice Kennedy’s opinion overflows with sentiment. It is sappy. (Kennedy’s sentiments, by the way, are in line with those of Mr. Conservative, Barry Goldwater, the classic example of a conservative who moved leftward in his advanced years).

Chief Justice Roberts’ opinion (and to a lesser extent some of the other dissents) is full of reminders that the case must be judged as a matter of constitutional law, not sentiment. Roberts, though, may be showing his age by writing (excessively, in my view) about how the Court’s decision will be “celebrated.” Twenty years ago, would he have done so?

The Obamacare cases also were arguably influenced by age. Forget about what Justice Scalia calls the the Chief Justice’s “sommersaults of statutory interpretation.” In my view, Roberts’ opinions are really about caution. In the first case (on the individual mandate), he was at pains not to overrule the legislature. In the second (on subsidies), he was desperate not to upset the health insurance market.

Caution is an attribute associated with advanced age.

Sentimentality also favored the result in the subsidy case. This challenge to Obamacare called on the Court to take away subsidies from millions of relatively low income individuals. Many would have opted not to buy health insurance, thus falling back into the ranks of the uninsured. Many others might have bitten the bullet and suffered financially as a result.

Thus, rejecting this challenge to Obamacare was the sentimental move, as well as the cautious one. Given their ages, it is not surprising that only three of the five Republican-appointed Justices were willing to place the cold words of the statute ahead of sentiment and caution.

The back-to-back-to back losses in the Supreme Court last week has conservatives demanding better vetting of judicial nominees. Better vetting is, of course, better than unimproved vetting.

But if I’m right about the drivers of the less than fully satisfying performance of Republican-appointed Justices, then vetting has significant limitations.

Republicans can be more aware of the particular strand of conservatism to which a potential nominee adheres. But they can’t be prescient about how a particular strand will play out ten or twenty years down the road. Will conservatives feel better served by Justices who tend to defer to the legislature or ones who do not? The answer probably depends on which way the political winds blow in the distant future.

As for aging, there is no cure. Republicans can delay its impact in this context by nominating younger Justices, but there are limits to how young a Supreme Court Justice can, acceptably, be. Moreover, the younger the nominee, the smaller the track record, and therefore the harder he or she is to vet.

Let’s not, however, allow a terrible week to obscure the progress conservatives have made in vetting judicial nominees at all levels. Justice Alito has been everything conservatives hoped for.

And Chief Justice Roberts is no Justice O’Connor, Souter, or Kennedy — at least not yet. He voted with Justices Scalia, Thomas, and Alito in two of the three big cases last week. His other main departure from conservative orthodoxy this term occurred in a case regarding the financing of elections for judicial office — a case that I think probably was correctly decided.

Still, there is no disputing that the Chief Justice is no longer the jurist conservatives hoped for. This development adds resonance to John’s call for conservatives to view the Supreme Court as the super-legislature it largely is.

But let’s remember that liberals will always have the advantage when it comes to selecting the super-legislators.