Media playing to chauvinist men by portraying women as violent are skewing statistics, says professor.
A world expert on women and crime says a "moral panic" about violent girls is seeing more of them arrested and jailed, even though evidence suggests they are actually becoming less violent.
Meda Chesney-Lind, a Hawaiian women's studies professor who is in Auckland for a criminology conference, said the panic was fuelled by media firms appealing to chauvinist men by portraying women - especially black women - as violent.
The story was reinforced by "zero tolerance" policing policies which led to more females being arrested for minor crimes that would once have been dismissed with a warning.
"I looked at juvenile robbery because it seemed to be hugely increasing," she said. "It turned out the average amount being stolen was $2.50."
As a result, girls had increased from 21 per cent of juvenile arrests in 1983 to 30 per cent last year in the United States, and from 19 per cent in 2001 to 22 per cent in 2010 in New Zealand.
Yet US surveys showed that both girls and boys were actually becoming much less violent. Only 23 per cent of girls and 39 per cent of boys had been in a fight in the past year in 2009, compared with 34 per cent of girls and 50 per cent of boys in 1991.
New Zealand surveys of secondary school students by the University of Auckland's Youth 2000 unit also found slight reductions in fighting between their two surveys published so far in 2001 and 2007.
Only 12.2 per cent of girls and 26.2 per cent of boys had been in a serious physical fight in the past year in their 2007 survey, compared with 14.5 per cent of girls and 27.9 per cent of boys in 2001.
"This 'crime wave' doesn't exist," Professor Chesney-Lind said.
But news media played up girls' violence in a "backlash" against feminism which appealed to men, choosing images of young women holding guns or in other violent poses.
"There is nothing accidental about any of these media images. Girls peering over the barrel of a gun is just hugely popular," she said.
Most of the photos were of black women. A study of how female drug offenders were depicted in 17 newspapers found that white women were typically shown at home with their families talking about how they had reformed, whereas black and Latino women were portrayed as "there she goes again, we let her out, she's back on drugs".
"But even false moral panics have real consequences," Professor Chesney-Lind said. "Between 1991 and 2003 detentions of girls went up 98 per cent, while detentions of boys went up only 29 per cent.
"African-American girls represent nearly half of those in detention. Seven out of 10 cases involving white girls were dismissed, compared with three out of 10 cases involving African-American girls."
Women have also increased as a proportion of New Zealand prisoners, though more modestly - from 5.1 per cent in 2001 to 6.1 per cent in June.
Nelson anthropologist Donna Swift agreed that there was not enough evidence to say whether girls' violence was actually increasing. But she said there was a lack of gender-specific programmes to address it.
"We can't really say that it's growing," she said. "We can say that what we have got, we are not addressing in a good way."