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The allegations of sexual misconduct against Donald Trump are new and old all at once. With accusers emerging in public for the first time, the stories themselves are clearly new to the public. The underlying allegations are old, however, often involving events from decades ago.

Trump insists that is crucial, proving his accusers have changed their stories.

At a rally last week, Trump questioned why People magazine's Natasha Stoynoff didn't mention his advances in a 2005 profile she wrote about him.

"Why didn't she write what she said happened before she wrote the story? Why didn't she put it in the story?" he asked.

"Why didn't she do it 12 years ago?" Trump continued, adding, "She's a liar."

Some of Bill Clinton's defenders have made the same point. Former Clinton aide Sidney Blumenthal, for example, in his 2002 book insisted that a woman who accused Clinton of rape couldn't be "a reliable witness" after she changed her story.

According to legal experts, however, a delay in reporting or publicizing a sex crime does not mean it didn't occur.

The data actually indicate the opposite — most sex crimes are never reported in the first place.

One of the most comprehensive studies on the issue, a 2013 federal examination of crime data over 10 years, found that only 32 percent of rapes and sexual assaults were reported to police.

Scott Berkowitz, who heads the Rape, Abuse & Incest National Network, or RAINN, a national anti-sex-crime group, said many victims "do everything they can to put this behind them."

Related: 11th Woman Accuses Donald Trump of Inappropriate Sexual Behavior

"Reporting to police can lead to a several-year process of adjudication," he said.

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While most victims make no formal report, others tend to delay before reporting their experiences or discuss them widely.

"It is very common for victims to delay in reporting a sexual assault," said Kelly Moe Litke, director of prevention programs for the Wisconsin Coalition Against Sexual Assault.

Legal experts point to a wide range of reasons victims don't report.

While judges and jurors can weigh many factors to assess an accuser's credibility, judicial guidelines note that people may delay reporting for reasons that have nothing to do with the truth or falsity of the allegation.

A 2015 guide for Pennsylvania judges (PDF), for example, lists a litany of reasons for delay, including "fear of retaliation by the offender, fear of not being believed, fear of being blamed for the assault, fear of being 'revictimized' if the case goes through the criminal justice system, belief that the offender will not be held accountable, wanting to forget the assault ever happened, not recognizing that what happened was sexual assault, shame, and/or shock."

Some of those risks are more pronounced when making a public accusation against a powerful or famous person.

Berkowitz argued that a sad reality for victims, as a practical matter, is that "the challenges of reporting often outweigh the benefits of getting justice."

Public opinion can also present a tough hurdle for accusers, both in and out of the courtroom.

Studies indicate that people "frequently misconstrue" delays as a sign a "victim is not being truthful" about an attack, according to the National District Attorneys Association (PDF).

"People believe there is a connection between the timeliness of the report and the truthfulness of the report," said Ian Henderson, legal director for the Wisconsin Coalition Against Sexual Assault.

At the same time, prosecutors say, a reporting delay does require examination.

Related: Trump Denials Are 'Rape Myths and Stereotypes 101,' Advocates Say

"When there's a delay, you certainly have to ask them why was there was a delay," former prosecutor Matthew Galluzo said, in order to find out "what was the reason it took you so long to go public with this."

"But because it's so typical to not make the report right away — or to never make the report at all — certainly just the fact that there's been a delay in these accusations does not prove that they're false," said Galluzo, who worked for the sex crimes unit of the Manhattan district attorney's office.

Apart from the particular challenges of prosecuting sex crimes, however, state and federal laws are generally tilted against delayed prosecutions of almost all crimes.

In a tradition that dates back centuries, courts and legislatures are generally skeptical of pursuing very old crimes because there is a higher risk of wrongful convictions, mistaken memories and other forms of bias.

Most criminal accusations have relatively short deadlines for prosecution. Federal prosecutors must generally press charges within five years of an alleged offense, for example, after which a "statute of limitations" prohibits any indictment.

That deadline is eliminated only for the most serious felonies, such as murder, and there has been a growing movement to include rape. Congress took that step in 2006, eliminating the prosecution deadline for sex crimes, and states have passed various exceptions, from extending the deadline to accept new DNA evidence to eliminating the deadline for felonies like rape or sex crimes against minors.

Each state sets its own rules; in a 2013 tabulation (PDF), the National Center for Victims of Crime found that eight states had eliminated the deadline for sex crimes.

Even when prosecution is legally barred, experts say, victims sometimes choose to speak out later based on the public environment.

"We often see cases of when a prominent teacher, doctor or other community figure is accused of sexual assault, news of that will often prompt others to come forward," said Berkowitz of RAINN. "They feel they are now more likely to be believed."

Zachary Schrieber contributed research for this report.