The memory of the accuser — especially if there are inconsistencies — is also often tested in court. But memory experts say inconsistencies are common and understandable in sexual assault cases. Jim Hopper, an independent consultant and teaching associate in psychology at Harvard Medical School, said that many sexual assault trials foundered because few jurors or police officers understood either the neurobiology of memory or how best to question a trauma victim.

“We know that stress and fear can have effects on how memories are encoded and stored,” he said. Typically, he explained, a victim will have a vivid memory of the onset of an attack because the brain is flooded with such stress hormones as adrenaline, which can temporarily enhance the ability to take in information. It may be easier to recount the onset of the attack than what happened before or after, because the brain assigns less emotional significance to those events. (In the Cosby case, Andrea Constand testified that she was given a drug, although the effects on her short- or long-term memory are unclear.)

The investigation process itself can also distort memory, Mr. Hopper said, as investigators often treat sexual assault accusers like criminal suspects, which can heighten fear and impair memory. “This sets them up for credibility problems later on,” he said, though he stressed that he did not know the specifics in the Cosby case.

When Mr. Hopper trains police, he coaches them to ask accusers open-ended and not leading questions. He also reminds them that after a shooting or a firefight, police officers and members of the military often render inconsistent or fragmentary accounts of what happened. Programs training the police in how to treat sexual assault victims have been adopted in such cities as San Diego; Austin, Tex.; and Philadelphia.

But giving up on the criminal system is not the answer, said Elizabeth Schneider, a professor at Brooklyn Law School. Over time, she said, the evolving understanding of rape has produced criminal convictions for marital rape, once considered impossible; rape when the parties have had consensual sex before; and rape when the parties know each other. About 68 percent of rape cases end in convictions, on a par with burglary cases, according to the Bureau of Justice Statistics. But legal experts said that only a fraction of sexual assault cases were ever brought to trial and that it was still much harder to win convictions when the parties know each other.

Professor Schneider proposed some possible remedies. When jurors are questioned during voir dire, and when judges give their instructions to the jury members before they deliberate, jurors could be educated about how rape and sexual assault victims often behave; the vagaries of memory that Mr. Hopper described; and the fact that it’s not unusual for a woman to continue contact with someone who assaulted her, particularly when the man could influence her career.