Washington ? Recognizing that a man’s home is, indeed, his castle, the Supreme Court said Wednesday that police can’t search a house without a warrant if one occupant invites them in but another objects to their presence.

Writing for a sharply divided court, Justice David Souter said the ruling’s logic was drawn from common social courtesies. The vote in the case was 5-3; Justice Samuel Alito, who wasn’t on the court when the case was argued, didn’t vote.

“There is no common understanding that one co-tenant generally has a right or authority to prevail over the express wishes of another, whether the issue is the color of the curtains or invitations to outsiders,” Souter wrote. Under the Fourth Amendment, which protects against unreasonable searches, police who come knocking without a warrant can’t bypass consent any more easily than a salesman or other guest could, he said. Justices John Paul Stevens, Anthony Kennedy, Ruth Bader Ginsburg and Stephen Breyer joined Souter in the majority.

The ruling could have wide-ranging implications for authorities, who regularly confront the question of how and when someone has sufficiently waived his or her privacy rights to justify a search.

The ruling also inspired the first written dissent from Chief Justice John G. Roberts.

Roberts accused the court of falsely reading a social assumption into the Constitution – “that an invited guest who arrives at the door of a shared residence, and is greeted by a disagreeable co-occupant shouting ‘stay out,’ would simply go away.”

Roberts said that different social situations could produce different outcomes.

“Such shifting expectations are not a promising foundation on which to ground a constitutional rule,” Roberts wrote. Justices Antonin Scalia and Clarence Thomas also dissented.

Roberts also disagreed with another of the court’s assumptions: that a person who chooses to live with someone else has the same expectations of privacy as a person who chooses to live alone.

“Even in our most private relationships, our observable actions and possessions are private at the discretion of those around us,” Roberts wrote. Someone who lives with others “assume the risk that those who have access to and control over his shared property might consent to a search.”

The case arose from a domestic dispute involving spouses Scott and Janet Randolph in Americus, Ga. When police showed to resolve things, Janet Randolph told them that her husband had drugs in their home and agreed to let the officers inside to look for evidence.

Scott Randolph objected, but the police went ahead on Janet Randolph’s word; she led them to drug paraphernalia in the couple’s bedroom.

Scott Randolph later tried to have the evidence tossed out because he said the search was illegal. A lower court agreed with him and the justices were asked to decide the matter finally.