IF nine South Hadley, Mass., high school students  seven of them girls  are proved to have criminally bullied another girl who then committed suicide, as prosecutors have charged, they deserve serious legal and community condemnation.

However, many of the news reports and inflamed commentaries have gone beyond expressing outrage at the teenagers involved and instead invoked such cases as evidence of a modern epidemic of “mean girls” that adults simply fail to comprehend. Elizabeth Scheibel, the district attorney in the South Hadley case, declined to charge school officials who she said were aware of the bullying because of their “lack of understanding of harassment associated with teen dating relationships.” A People magazine article headlined “Mean Girls” suggested that a similar case two years ago raised “troubling questions” about “teen violence” and “cyberspace wars.” Again and again, we hear of girls hitting, brawling and harassing.

But this panic is a hoax. We have examined every major index of crime on which the authorities rely. None show a recent increase in girls’ violence; in fact, every reliable measure shows that violence by girls has been plummeting for years. Major offenses like murder and robbery by girls are at their lowest levels in four decades. Fights, weapons possession, assaults and violent injuries by and toward girls have been plunging for at least a decade.

The Federal Bureau of Investigation’s Uniform Crime Reports, based on reports from more than 10,000 police agencies, is the most reliable source on arrests by sex and age. From 1995 to 2008, according to the F.B.I., girls’ arrest rates for violent offenses fell by 32 percent, including declines of 27 percent for aggravated assault, 43 percent for robbery and 63 percent for murder. Rates of murder by girls are at their lowest levels in at least 40 years.