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“That’s always the classic claim, that you’re blaming the victim, but that’s said by people who don’t understand,” Dr. Smith’s lawyer, Roy Black, said in an interview Tuesday. “The victim may not be on trial, but her testimony, her accusation, is on trial.”

A claim of consensual sex failed for boxer Mike Tyson, convicted in 1992 of raping an 18-year-old beauty pageant contestant. Mr. Tyson acknowledged having sex with the woman in an Indianapolis hotel room but said he didn’t force her. His lawyer argued that the woman filed charges out of anger at how Mr. Tyson had treated her after the encounter.

“It was a tough case,” the lead prosecutor, J. Gregory Garrison, recalled Tuesday. Especially in acquaintance-rape cases, with no third parties who can describe what happened, “it’s a question of persuasion.”

The woman’s torn clothes were shown during the trial, and a medical expert testified that the woman had sustained internal injuries that were inconsistent with consensual sex. But some of the key moments Mr. Garrison remembers were less about what was said than what was seen, as when audible gasps arose in the courtroom as the petite woman entered the courtroom to face the chiseled heavyweight champ. Mr. Tyson ultimately served about three years in prison.

It is usually difficult, if not impossible, to make a case without the accuser’s testimony, and “victims often don’t want to go through the nightmare that may result,” said Corey Rayburn Yung, a professor at the John Marshall Law School in Chicago who studies sex crimes.

The sexual assault case against NBA star Kobe Bryant was dropped after the woman told prosecutors she couldn’t take part in a trial. She had accused Mr. Bryant of raping her in a hotel room in Vail, Colo. Mr. Bryant apologized for his “behavior that night and for the consequences she has suffered,” while insisting the sex was consensual.

Prosecutors there said they were confident they could convict Mr. Bryant, but only with her cooperation.

“There are a lot of misconceptions by people. They think women make up sex assaults,” Mark Hurlbert, the prosecutor in Mr. Bryant’s case, said Tuesday. “I don’t know why … because it’s such a traumatic process, not only the attack but what the system does to these victims. The person who comes forward to testify is pretty sure of herself.”

Prosecutors and defense lawyers will scour for other evidence and indicators to back their side’s version of events. They might look at forensic evidence and injuries, if any; whether the accuser reported the alleged incident immediately, and how consistently he or she related it; how the defendant behaved in the immediate aftermath; whether the accuser is seeking money in a lawsuit.

“He says he didn’t do it, she says he did, so now whom do you believe?” said Mr. Black, Dr. Smith’s lawyer. “The only thing you can do is look at all of the little pieces of information, all the disparate things that at first don’t look important.”

Associated Press writer Colleen Long and researcher Julie Reed contributed to this report from New York.