WASHINGTON -- In 157 pages, the Supreme Court's decision defining a new, limited right of abortion is never explained simply. But there is one idea that apparently made all the difference -- an idea based as much on modern sociology as on law.
That notion, basically, is that privacy is vital to the institution of marriage in America today, for the partners, and for the marriage. And that idea led to the view that abortion rights -- if they are to exist at all -- must rest very much upon marital privacy, and less upon an open-ended individual right of privacy.
In the new ruling, marital privacy was crucial, because it is especially attractive to two justices whose votes were were necessary to the outcome: Justices Anthony M. Kennedy, a married man, and David H. Souter, a bachelor.
If it had not been for the high level of sensitivity Justice Kennedy and Justice Souter have about the need to protect marriage and the family from government intrusion, there would have been no majority of five. Without that, they very likely would not have voted with Justice Sandra Day O'Connor to keep Roe vs. Wade partly intact.
Through the years, Justice O'Connor has been in favor of some abortion right, though less than Roe vs. Wade had established. She thus has long seemed available to vote to retain a version of an individual woman's right to end a pregnancy.
She and Justices Kennedy and Souter, once they had made the commitment in recent weeks not to overrule Roe, as the Bush administration had asked, formed the core of a potential majority for the decision that finally emerged.
They picked up at least some support -- enough to make the voting outcome 5-4 -- from the court's only remaining liberal justices, Harry A. Blackmun (the author of Roe) and John Paul Stevens.
It is a fair assumption that if Justices Blackmun or Stevens had tried to draft a majority opinion reaffirming Roe primarily on its original premise -- the right of individual privacy -- Mr. Kennedy and Mr. Souter would have shied away. Justice O'Connor might not have.
Individual autonomy, in general, as a basis for constitutional conclusions, is a concept that makes "moderate conservatives" like Justice Kennedy and Justice Souter quite uncomfortable. They seem to fear where it would lead, making a constitutional matter out of a whole range of choices about individual autonomy.
Without either of those two justices, there could have been no majority to salvage any part of Roe vs. Wade. Four other justices, more conservative than they, were voting to overturn Roe outright, and those four were unyielding in that commitment to the final vote. The loss of any other justice to the cause of maintaining some right to abortion could have scuttled Roe altogether.
The concept of privacy within marriage goes explains much of the underlying theory of the majority opinion written jointly by Justices Kennedy, O'Connor and Souter, and to explain at least some of the more important parts of the ruling on particular restrictions in the Pennsylvania anti-abortion laws.
But that does not mean that the resulting decision protects only abortion rights for married women. The Constitution itself, with its guarantee of legal equality, could not be the source of a right for women only if they were married.
But that is the strongest premise for much of the ruling because it is tied directly to the unfolding in modern times of the entire concept of a constitutional right of privacy.
The decision that emerged Monday represented a distinct shift from Roe vs. Wade, on the overall privacy question. The majority reaffirmed generations of Supreme Court precedent establishing an expanding right of privacy -- the bulk of those decisions keyed to maintaining privacy in marriage.
Those decisions protect everything from mixed-race marriages, to the use of birth control among married couples, to the choice of how to educate one's children.
Roe vs. Wade is an outgrowth of those background decisions on marital privacy, but it went much further than those ever had gone to devise a right of privacy based on individual autonomy -- the woman's right to control her own physical destiny.
When Mr. Souter's nomination was before the Senate Judiciary Committee two years ago, and he was pressed repeatedly for his views on abortion and on the right of privacy, he did not hesitate to say he favored "marital privacy." But that is as far as he would go, on privacy rights in general, or on abortion as a privacy right in particular.
He had never had a chance, until Monday, to cast a vote on an abortion case, to give his views on privacy the most stern test that a judge can face.
Mr. Kennedy, too, has left no doubt of his quite strong commitment to marital privacy. Almost from the time he joined the court four years ago, he has made it clear repeatedly that he was worried that if Roe vs. Wade were overruled, past precedents on marital privacy might have to go, too.