When police arrived on the scene of a fatal stabbing last week in Brooklyn, N.Y., they were stunned by what they saw. The victim, an 11-year-old girl, lay crumpled on the floor, the front of her "Dora the Explorer" T shirt bloodied. The weapon, a steak knife, was in the kitchen sink. And the perpetrator, visibly upset and clinging to her mother, police say, was a little girl in a ponytail, only 9 years old. A few days later, she stood in white socks and shiny black dress shoes before a judge, listening as her lawyer entered a plea of not guilty.

The tragic event, which took place after the girls came to blows over a pink rubber ball, was a sad reminder that children can possess the same brutal instincts as adults. But for experts on youth crime, the killing was another instance of what they view as a burgeoning national crisis: the significant rise in violent behavior among girls. According to the FBI's Uniform Crime Report, the number of girls 10 to 17 arrested for aggravated assault has doubled over the last 20 years. The number of boys arrested for weapons possession rose 22 percent between 1983 and 2003, while the number of girls increased by a whopping 125 percent. Today, one in three juveniles arrested for violent crimes is female. "Girls are not what people think they are," says Dr. Howard Spivak, director of Tufts University Center for Children and coauthor of a new book, "Sugar and Spice and No Longer Nice: How We Can Stop Girls' Violence." "The change in girls' behavior is overwhelming."

A quick look at recent headlines is overwhelming indeed. On April 20, a 17-year-old from Lexington, Mass., allegedly slashed open the neck and face of another girl with a bottle of Twisted Tea. The next day, three teenage girls in Ayden, N.C., were charged with first-degree murder for participating in a drive-by shooting that left a 10-year-old boy dead. On May 3, a 17-year-old from Chicago was stabbed in her left breast and right armpit; a 16-year-old female classmate has been charged. And the teen daughter of former "Law & Order" star Dianne Wiest was recently arrested in Manhattan with two girlfriends for allegedly roughing up a male classmate and stealing his iPod. A court hearing is scheduled.

Schoolyards, where boy bullies once reigned supreme, are increasingly arenas for skirmishes between girls. "There are actually more physical girl fights now than between boys," says Bill Bond, a former school principal in Paducah, Ky., who travels the country studying safety issues for the National Association of Secondary School Principals. "I was just on a Cheyenne reservation yesterday and the principal said he had had one fight this year between boys and six between girls." Jennifer Clayton, 14, was beaten up in May by three other girls as she walked home from her school in Guelph, Ont. "I could hear them saying, 'Punch her in the face'," she told the local newspaper.

Jennifer Orangio, the 18-year-old slashing victim in Massachusetts, says that when she came upon her boyfriend hanging out with an ex in the school parking lot, the heat of her own reaction took her by surprise. Orangio went up to the other girl, Jamie Pelletier, and pushed her. Pelletier "threatened to smash a bottle over my head... I was, like, 'Go ahead, do it!' And she did it." Pelletier, 17, now faces felony charges of assault and battery with a dangerous weapon (to which she pleaded not guilty). She declined to comment.

Part of this spike in violence is related to evolving sex roles. Historically, boys have received messages from the culture that connect masculinity with physical aggression, while girls received opposite messages, encouraging passivity and restraint. Now girls are barraged with images of "sheroes"--think Sydney Bristow on ABC's "Alias" or Uma Thurman's the Bride in "Kill Bill: Vol. 2" --giving them a wider range of role models and tacit permission to alter their behavior. Accordingly, says Spivak, some girls have "shifted from internalizing anger to striking out."

The women's movement, which explicitly encourages women to assert themselves like men, has unintentionally opened the door to girls' violent behavior. "I was at a JV lacrosse game, watching my granddaughter. We cheered like hell because she was being aggressive on the field," says Joan Jacobs Brumberg, professor of history, human development and gender studies at Cornell. "I don't want to blame women's liberation for violence among girls," cautions Brumberg, but "traditional femininity and passivity are no longer valued in young females." James Garbarino, professor of human development at Cornell, puts it more bluntly. "We rely on boys to get out there and block a football, go in the Army and defend the country, carry guns and be cops. One of the side effects is that some boys take [physical aggression] too far." Now that girls have the same opportunities, he says, they can encounter the same blurry boundaries.

Research suggests that the best predictor of violent behavior, however--for girls and for boys--is not hours logged playing videogames or competitive pressure, but firsthand exposure to violent behavior. And social scientists warn that the number of children who see guns, fights and other kinds of physical abuse on a day-to-day basis is on the rise. "Violence in girls, like violence in boys, is really rooted in the individual and the individual's situation. I don't think you can blame the culture entirely for this phenomenon," says Brumberg.

After Ella Speight's 17-year-old daughter was attacked by a 16-year-old classmate last month, she spent hours in the hospital, tending to her child. Speight says she isn't angry: she prays for the assailant and even embraced the girl's mother when they met in court. "My heart hurts for her family," says Speight. "I know her mother didn't send her out to do that." Sugar and spice and everything nice: maybe Speight's forgiving nature represents an ideal that even boys can aim for.