Washington ? The number of rapes per capita in the United States has plunged by more than 85 percent since the 1970s, and reported rape fell last year even while other violent offenses increased, according to federal crime data.

This seemingly stunning reduction in sexual violence has been so consistent over the past two decades that some experts say they have started to believe it is accurate, even if they cannot fully explain why it is occurring.

In 1979, according to a Justice Department estimate based on a wide-ranging public survey, there were 2.8 rapes for every 1,000 people. In 2004, the same survey found the rate had decreased to 0.4 per thousand.

Many criminologists and victims’ advocates say these numbers could be a statistical mirage, because rape is still underreported and poorly understood. But others say they are convinced that there is real improvement and that a devastating crime has been receding.

“Overall, there has clearly been a decline over the last 10 to 20 years,” said Kim Gandy, president of the National Organization for Women. “It’s very liberating for women, in terms of now being able to be more free and more safe.”

By all accounts, rape is still one of the most underreported crimes. Several decades after the establishment of rape crisis hot lines, greater sensitivity toward rape victims by police and prosecutors, adoption of policies by news organizations to not identify victims and limitations on how much a victim’s sexual history can be placed in evidence during trial, the Justice Department estimates 61 percent of rapes and sexual assaults are still not reported. But that is down from 69 percent in 1996, and experts say the trend remains downward.

Not everyone is convinced things are getting that much better. Many who work with rape victims say they do not believe there has been a widespread decline in the number of attacks. Instead – despite the years of attempted outreach to rape victims – they say the crime might be as hidden now as ever.

“If there’s been a change, it’s been a very small change,” said Dean Kilpatrick, director of the National Crime Victims Research and Treatment Center in Charleston, S.C. He said recent high-profile rape cases such as those involving Duke University lacrosse players and basketball star Kobe Bryant might have persuaded rape victims to stay silent because of public scrutiny on the accusers’ private lives and sexual history.