Nationally, police apprehensions of girls for "acts intended to cause injury" have almost doubled in the past 16 years, from seven to 12.3 per 1000 girls in the 14-16 age group, while apprehensions of boys have been stable at between 18 and 20 per 1000 boys.
Dr Swift said this was partly because girls' violence may have been ignored in the past, but she believed girls were also becoming genuinely more violent.
"Look at the media," she said. "The sexualised kick-ass girl is is highlighted as a girl who can throw her own weight around and attract male attention and be aggressive, not deal with emotion. She lacks empathy. She is there for herself."
Skylight Trust deputy head Tricia Hendry, co-author of a new book called The Anger Toolbox, launched yesterday, said young people were becoming angry because of growing stress in modern society.
Dr Swift also found effects of family breakdown in 100 intensive interviews with girls who had been involved in violent behaviour. One girl told her: "The majority of me and my friends don't have father figures."
But Dr Swift found that the main reason for most girls' fighting was "competition over boys' attention".
She said many girls adopted a "tough girl" self-identity as a way of feeling good about themselves because they didn't get approval from their parents or teachers.
"It was commented that boys found girl fighting entertaining and sexually stimulating," she said.
Dr Swift said her research showed that girls' violence was caused by quite different factors than boys' violence and needed gender-specific interventions.
She has run a pilot programme with girls for Child, Youth and Family Services but has not been able to get funding to extend it.ON THE WEB
* www.skylight.org.nz
* www.thegirlsproject.org.nz
