There was a time when police officers handled a domestic violence call by telling angry men to take a walk and cool off. They still do, but the walk is straight to jail. And increasingly, it is the woman who takes the hike.

Police in at least 24 states now receive training in how to decide who is the "primary aggressor," a term that does not necessarily mean the person who struck the first blow or even caused the most damage, according to the National Council of Juvenile and Family Court Judges.

"Primary" means "most significant." The definition requires officers on the scene to go through a long checklist of indicators, including a history of violent or coercive behavior.

Backers of the "primary aggressor" law hoped it would reduce domestic violence and the number of women arrested for defending themselves.

In 1987, women were arrested in 5 percent of California's domestic violence cases; that rate had risen to 15 percent by the time the state passed its primary aggressor law in 1997.

Last year, as overall domestic violence arrests declined in California, the percentage of women arrested rose still further, to 16 percent: 9,373 arrests compared with 47,519 for men.

Just why more women are being arrested is unclear. Social scientists and police departments are stumped. But some possibilities are that women are being more aggressive, that women are beating other women, and that male victims are increasingly likely to come forward and be believed by officers.

Another possible reason is that there are more female police officers, said Katharine Killeen, director of the California District Attorney Association's Violence Against Women Project.

"They don't just let a woman go the way some men cops might," she said. "Also, men are learning how to work the system better and bring charges."

Women's groups campaigned heavily for primary aggressor laws, which were designed to prevent battered women from also having to go through the trauma of being arrested for fighting back.

New York Gov. George E. Pataki (R) made note of that in 1997 at the signing ceremony for his state's law. "Women who are the targets of domestic violence should not be victimized again by being arrested simply for defending themselves," Pataki said.

That is one reason the increase in the arrests of women under primary aggressor laws is surprising.

"This is geared to the prosecution of men," said Deputy Public Defender Lidia Stiglich, who handles arraignments in San Francisco's domestic violence court.

Whoever calls the police first wins, she said. And that is usually the woman. "If you are a man, you are toast," she said.