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On the opening day of the prosecution case against Bill Cosby, prosecutor Kristen Feden called out the name of an expert witness to lead off Cosby’s retrial on sex assault charges.

No, it wasn’t Andrea Constand, the lanky former pro basketball player and main witness against Cosby, who alleges that he drugged and sexually assaulted her.

It was a self-assured forensic psychiatrist named Barbara Ziv, who testified about “rape myths” — widely believed misconceptions about the behavior of victims of rape and sexual assault, such as expecting women to immediately file police reports and to cut off contact with the men who attack them.

“Most common knowledge about sexual assault is wrong,” Ziv said.

In a sense, the Cosby retrial, now nearing the two-week mark, is shaping up as a referendum on rape myths, a test both of their grip on the mind-set of the public and of the notion that there may be exceptions to their tenets. Prosecutors, who rested their case Thursday, are betting on jurors absorbing the concept and applying it to Constand’s testimony, particularly her behavior after she says Cosby assaulted her in 2004.

The core of the defense case is a portrayal of Constand as a liar and extortionist. But Cosby’s attorneys are also angling to raise doubts that she is a true victim by pounding her with arched-eyebrow questions about the same sorts of behaviors that Ziv described as commonplace for women who have been sexually assaulted.

A strategic shift

Choosing Ziv represented a major tactical shift by prosecutors. In Cosby’s first trial, which ended with a hung jury last June, they opted for a more dramatic kickoff, calling a witness who sobbed about Cosby allegedly drugging and sexually assaulting her at the swanky Bel-Air Hotel in Los Angeles in 1996. This time, they opted to first educate the seven-man, five-woman jury about rape myths.

Unraveling rape myths, as told by Ziv

1. While being cross-examined, Ziv testified that no more than 7 percent of sexual assault allegations are false, and she thinks the number could actually be as low as 2 percent.

2. She explained that people generally believe sexual assault is committed by strangers when, in fact, 85 percent of victims know their attackers.

3. Citing U.S. government statistics, she said fewer than 30 percent of sexual crimes are reported to police.

4. She also advised jurors that the public often wrongly thinks that sex-assault victims do things to make themselves vulnerable to attack.

5. And, crucially for the Cosby case, she testified that victims are often fuzzy about details, frequently wait for long periods to report their crimes and typically maintain contact with their attackers to try to “make sense” of what happened to them.

How Ziv might help Constand

A key potential problem with Constand’s accounts is that she told a police investigator that she was assaulted after eating dinner with Cosby and a group of Philadelphia educators at a Chinese restaurant in March 2004. She later changed her account, saying the assault happened two months earlier on an evening when she arrived at Cosby’s house with an empty stomach and he insisted she drink an old-vintage wine.

She testified that she didn’t contact police until a year later when she’d moved back to Canada and began screaming at night from persistent nightmares.

That is normal behavior, according to researchers such as Ziv.

Once Constand went to police, her statements were fraught with inconsistencies large and small — a not uncommon occurrence with sexual assault victims still sorting through their trauma. After the Chinese dinner, she explained that she had gone to Cosby’s home to confront him. But, instead, she left with her questions unanswered, wondering exactly what happened — and why.

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