Cosby Verdict Is Hailed a Breakthrough. Here's Why It Was an Anomaly.

The guilty verdict against Bill Cosby was hailed by some as long overdue.

Not just because his victim, Andrea Constand, first reported that he had sexually assaulted her 13 years ago, but also because, as a lawyer for other Cosby accusers put it, “women were finally believed.”

Getting there required surmounting significant obstacles: a victim who waited a year to report the crime and gave two different dates for the attack, a prosecutor who declined to bring the case, a new district attorney who raced against the statute of limitations and, finally, a first trial in which the jury could not agree.

That was before a series of revelations that caused a seismic shift in public awareness of sexual assault by powerful men. And this is after. But those who know the legal system best say the verdict is not the bellwether that many would like. Criminal convictions in sexual misconduct cases where the victims and the accused knew each other will remain exceedingly hard to get.

Neither Harvey Weinstein, the former Hollywood mogul, nor Russell Simmons, the music producer, has faced criminal charges, despite investigations into allegations against each of them. James Toback, a film director who has been accused by scores of women of sexual misconduct over decades, will likely never face arrest because the statute of limitations has expired.

“The Cosby trial is an anomaly,” said Barbara Ashcroft, a former sex crimes prosecutor in Montgomery County outside Philadelphia, where Mr. Cosby was tried. “The reality for sex crimes prosecutions against victims of acquaintance sexual assault, even post-Cosby conviction, is the cards are stacked against them.”

On Friday, one of Mr. Cosby’s publicists, appearing on “Good Morning America,” compared Mr. Cosby’s case to that of Emmett Till, a boy who was lynched in 1955 on suspicion that he had flirted with a white woman, who later denied that he had done so.

Ms. Constand broke her silence since the verdict on Twitter, saying, “Truth prevails.”

But truth has an arduous path.

Delayed reporting by victims makes the collection of physical evidence nearly impossible and can trigger the protection of statute of limitations laws. The police, prosecutors and juries may also question why they waited so long.

Alcohol or drugs are often involved in such cases, which can affect the memory, as can trauma. The presence of mind-altering substances can encourage juries to focus on the actions of the victim, instead of the accused.

Prosecutors may be reluctant to bring charges in a case that they think they cannot win. And even if a case reaches the courtroom, juries may accept that sex occurred but question whether it was consensual, asking for physical evidence of resistance or injury that may not exist.

Taken together, the legal standard to win a criminal conviction — beyond a reasonable doubt — is usually insurmountable, analysts say.

“Jurors often say that they did not convict not because they did not support the defendant, but because the state did not meet this very high standard of proof,” said Ken Broda-Bahm, a jury expert with the law firm Holland & Hart. “On the other hand, the public standard is, ‘What do you think happened?’ which jurors are specifically asked not to do.”

In New York, the Manhattan district attorney’s office is investigating a claim by Paz de la Huerta, an actress, that Harvey Weinstein raped her on two different occasions in 2010, and an allegation that Mr. Weinstein forced an acting student, Lucia Evans, to perform oral sex during a business meeting at his office in 2004.

Cyrus R. Vance Jr., the district attorney, has been criticized for failing to act on an earlier complaint against Mr. Weinstein, and the police have said that they developed a strong criminal case based on Ms. de la Huerta’s account. But Mr. Vance has said his office is still gathering evidence.

Earlier this month, Mr. Vance appointed to the investigation a new prosecutor who is known for winning convictions in cases with little physical evidence. But in 2011 the prosecutor also recommended throwing out a sexual assault indictment against Dominique Strauss-Kahn, the former head of the International Monetary Fund, after the reliability of the complainant, a hotel housekeeper, came into question.

In Los Angeles, prosecutors are investigating a claim that Mr. Weinstein raped a woman in a hotel room in 2013, according to the Los Angeles Times. The woman did not immediately report the crime, so no rape kit was taken, the paper reported. Mr. Weinstein is also under investigation for a sexual assault in London.

In all, Mr. Weinstein has been accused of rape, assault or sexual misconduct by more than 80 women. Representatives for Mr. Weinstein have denied the criminal allegations, saying that any sex acts were consensual.

Russell Simmons, a music industry executive, has also been accused of sexual assault and other crimes by at least 10 women. He has denied having sex that was not consensual.

Three women who said their assaults took place in New York — Tina Baker, Drew Dixon and Toni Salle, each of whom said Mr. Simmons raped them in incidents between 1988 and 1995 — said they spoke with a detective from the N.Y.P.D.’s cold case squad, a part of the Special Victims Unit. However, the detective cautioned them that their allegations could not be prosecuted because the incidents occurred outside the statute of limitations. The detective told them their accounts would be kept on file for potential use should a more recent case surface.

The statute of limitations for sexual assault in California — where a number of the allegations against Mr. Simmons and Mr. Weinstein have been made — had until recently been 10 years.

But in the wake of the allegations made against Mr. Cosby, California eliminated statute of limitations protections for sexual assaults in 2016. The new rule applies only to crimes committed after Jan. 1, 2017. New York abolished the statute of limitations on rape in 2006, but there is still a five-year limit for other types of felony sexual assault and two years for misdemeanors such as groping.

Earlier this month, the Los Angeles County district attorney’s office announced that Mr. Toback, 73, who has been accused of sexual misconduct by scores of women over decades, would not be charged in five separate investigations because the incidents were beyond the statute of limitations.

The Los Angeles Times has reported that hundreds of women had come forward to complain about Mr. Toback, who has denied wrongdoing.

Even without statute of limitations laws, an extended period between a crime and a report makes cases very difficult to prove, said Ms. Ashcroft, the former sex crimes prosecutor. And victims themselves are often regarded as unreliable, she said.

“Victims are vulnerable and because they are damaged, they often do not make good witnesses,” said Ms. Ashcroft. “The passage of time also often blurs their memories and brings inconsistencies. And in a trial in which there are only two people who know what happened, that is the cornerstone of who you believe.”

In assault cases lacking physical evidence or corroborating witnesses, Ms. Ashcroft said she would sometimes have to convince superiors to allow her to pursue charges.

Skeptical judges would pressure prosecutors to agree to plea deals “to get rid of pesky sex crime cases,” she said, or ask, “Why are you trying this case?”

“There are layers and layers of obstacles to bringing a sexual assault case,” Ms. Ashcroft said.

One critical difference between Mr. Cosby’s mistrial and his conviction was that in the second trial, Judge O’Neill allowed five women in addition to Ms. Constand to testify. Each said they believed Mr. Cosby had drugged and sexually assaulted them. In the first trial, only one other accuser had been allowed to testify.

Mr. Cosby’s lawyers are likely to raise the decision to allow the additional testimony on appeal, because ordinarily evidence of prior bad behavior is not permitted.

“Even though it is the first celebrity #MeToo case, it is not typical,” said Deborah Tuerkheimer, a law professor at Northwestern University and former prosecutor.

The law, Ms. Tuerkheimer said, responds slowly to social change: “Unless and until #MeToo has penetrated to every member of society, when you choose 12 people at random, there’s just no telling what’s going to happen.”