Until the 19th century, there was a charming little rule of thumb that applied to family life. A man was allowed to beat his wife as long as the stick he used was no wider than a thumb.

Perhaps this law was humanitarian when it was written, even protective. After all, a woman was kept under all sorts of thumbs in colonial America. Unless she married Paul Bunyan, this law may have kept the instrument of her torture down to an inch.

But it seems unlikely that, in real life, a colonial wife could call the authorities to report it when her husband resorted to force. No peace officer, unless he was suicidal, would enter a colonial household to measure the width of a stick.

In any case, we still live with the legacy of this rule of thumb. There is a lingering sense of social permission for family violence. Intimates assault each other with much less fear of reprisal than strangers.

Police, for their part, often feel trapped between demands and theories. They are criticized for "interfering" in a family squabble at all and criticized for not protecting victims. They are alternately told to act as social workers and then as police.

But now, the Police Foundation may make a difference. With funding from the National Institute of Justice and help from the Minneapolis police, Lawrence Sherman and Richard Berk have conducted an experiment to test three ways of handling domestic violence: mediating the fight, separating the couple and arresting the suspect.

The winner of the deterrence sweepstakes, it appears, is arrest. The men (mostly) who were arrested were measurably less likely to be violent again.

The early returns from the Minneapolis police, as Sherman emphasizes, are still tentative. The 328 cases have been followed for only six months. There are peculiarities to this study and this place. The suspects were mostly unemployed; the couples were mostly unmarried. In Minneapolis, arrest means 24 hours in jail.

Still, there is evidence here of a way to curb domestic violence. There is evidence that punishment may be the most effective deterrent. It may be better to deal with an abusive mate as a criminal than as a lover who just lost his temper.

Sherman, who has listened to the police express their reluctance to make arrests, is encouraged by the data. "The police often say they can't assess blame in domestic cases. They go on and on. 'Suppose she nagged him and he hauled off and hit her; whose fault is it then?' But I'm not sure this is as much an issue as preventing it from happening again."

He has a theory about why arrest is itself a deterrent, although none of these cases went to court. He calls it the empowerment theory. "Between men and women, the power balance is distorted in favor of the one who has the bigger muscles. When they call the police, women involve the police muscles and even up the power balance."

In fact, arrest works best as a deterrent when the woman herself, and not a neighbor, has called the police. As Sherman explained it, "If the arrest appears to be something the woman made happen, then it has a stronger effect."

But there may be a complementary reason arrest works in domestic cases. In every society, some acts of violence have been historically condoned by some. Rape is one. Domestic violence is another.

One of the extraordinary aspects of the New Bedford gang rape is that the alleged rapists were still in the bar when the victim returned with the police. They never expected to be arrested.

Domestic disputes have a similar pattern. In some cultures and homes and psyches, wife beating is still considered a right, and violence regarded as a private family matter. In such cases, arrest may be a greater deterrent because it redefines an accepted act as a crime like any other. The moral weight of society shifts abruptly from one side to the other. There is a moral balance of power that may be as important in human relationships as the physical.

This research, preliminary and incomplete, may give us a useful way to treat these intimate enemies. The new rule of thumb may be on the arresting arm of the law.

Copyright (c) 1983, The Boston Globe Newspaper Company