The assertion from forces against Brett M. Kavanaugh that false allegations of rape or attempted rape are extremely rare is rebutted by a number of studies, researchers say.

Liberals on TV and social media said repeatedly during the Senate confirmation process that only 2 percent of charges are lies — meaning there likely would be truth in the majority of such charges, such as that of Christine Blasey Ford, who accused now-Supreme Court Justice Kavanaugh of sexually assaulting her 36 years ago.

Brent E. Turvey, a criminologist, wrote a 2017 book that dispels this notion. His research, and that of two co-authors, cited statistical studies and police crime reports. One academic study showed that as many as 40 percent of sexual assault charges are false. Mr. Turvey wrote that the FBI in the 1990s pegged the falsity rate at 8 percent for rape or attempted rape complaints.

“There is no shortage of politicians, victims’ advocates and news articles claiming that the nationwide false report for rape and sexual assault is almost nonexistent, presenting a figure of around 2 percent,” writes Mr. Turvey, who directs the Forensic Criminology Institute. “This figure is not only inaccurate, but also it has no basis in reality. Reporting it publicly as a valid frequency rate with any empirical basis is either scientifically negligent or fraudulent.”

A recent study supports this assessment. The Pentagon issues an annual report on sexual assaults in the military. Nearly one-quarter of all cases last year were thrown out for lack of evidence, according to a report released in May.

“It appears that die-hard opponents of Kavanaugh have invented a narrative to imply that false accusations hardly ever happen,” said Elaine Donnelly, who directs the Center for Military Readiness.

“You see where they are going with this,” she said. “Any man who doubts Ford is hostile to women experiencing abuse, who make accusations truthfully 90 to 98 percent of the time. This is why hard data from the Pentagon, which shows rates of false accusations averaging 18 percent in annual reports since 2009, is important.”

Women’s advocates say that an unfounded case doesn’t necessarily mean the accuser was lying.

The National Sexual Violence Resource Center puts the false report rate at 2 percent to 10 percent.

“Research shows that rates of false reporting are frequently inflated, in part because of inconsistent definitions and protocols, or a weak understanding of sexual assault,” the center said.

Three women accused Justice Kavanaugh of sexual misconduct. During the confirmation process, the FBI looked at two of those complaints: Ms. Ford’s and one from Deborah Ramirez, a former Kavanaugh classmate at Yale University. The FBI, as well as Senate Judiciary Committee Republican staff, interviewed potential witnesses who didn’t back up either charge.

A third charge, Julie Swetnick’s accusation of gang rape, was deemed not credible by Republicans and wasn’t on the FBI interview list.

NBC News showcased Ms. Swetnick and reported that she provided the names of four witnesses. One was dead, one said she didn’t know Ms. Swetnick and two didn’t respond.

Ms. Ramirez and Ms. Swetnick refused the committee’s requests for interviews, according to Republican staffers.

Ms. Ford testified before the Senate Judiciary Committee on Sept. 27 that Justice Kavanaugh tried to rape her at a high school house party 36 years ago when he was 17. He denied the event ever happened. The FBI reported to the committee that agents interviewed supposed attendees, including Ms. Ford’s high school best friend. They all failed to corroborate that the party took place.

The Obama administration ordered the military to wipe out sexual crimes, leading the Pentagon to improve one of the most extensive sex-crime-tracking studies in the country from the Defense Department’s sexual assault prevention and response office. The campaign’s objective was to spur more personnel to report assaults — which they have.

The statistics on unfounded cases are contained in an appendix of 3,567 cases last year. Of those, 729 cases were dropped because of “insufficient evidence” that a crime was committed. Another 79 were deemed “unfounded” at the command/legal review level. The total — 808 — represents 23 percent of all cases that year.

Mrs. Donnelly said the Defense Department office, up until 2015, classified those two categories of cases — insufficient evidence during the investigation or at the command level — as “unfounded.”

She said the rate of unfounded complaints jumped from 13 percent in 2009 to 26 percent in 2016 and fell to 23 percent last year.

Mr. Turvey’s 2017 book, “False Allegations: Investigative and Forensic Issues in Fraudulent Reports of Crime,” looked at a range of bogus reporting, including on rape and sexual assault. He examined existing studies and police statistics.

“False reports happen, they are recurrent and there are laws in place to deal with them when they do,” he wrote. “They are, for lack of a better word, common.”

Mr. Turvey quotes a study by researcher Edward Greer, past president of the Association American Law Schools. He traced the one and only source for the “2 percent” assertion to a 1975 book, “Against Our Will: Men, Women and Rape,” which quoted statistics from New York City, not from across the nation.

Mr. Turvey cites 10 studies that debunk the 2 percent assertion in the U.S. and abroad.

“The power of any lie is equal only to our desire to believe it,” Mr. Turvey wrote. “Specifically, our need and eagerness to believe it. This is the problem with belief — which is accepting something as true or correct without proof.”

Sexual assault is a broad term that encompasses different types of penetration, attempted penetration and unwanted touching or attempts.

For statistical purposes, the Justice Department tracks rapes, attempted rapes and a threat of rape as one category. It keeps separate statistics on other types of sexual assaults.

It defines assault as “A wide range of victimizations, separate from rape or attempted rape. These crimes include attacks or attempted attacks generally involving unwanted sexual contact between victim and offender. Sexual assaults may or may not involve force and include such things as grabbing or fondling. It also includes verbal threats.”

Conservative commentator Michelle Malkin says the 2 percent number was one of the most frequent weapons unleashed against Justice Kavanaugh, who was confirmed and sworn in on Saturday.

“The truth is that number has no documented empirical basis,” Ms. Malkin said in a Twitter video.