The crime few people talked about five years ago is out in the open, and it looms surprisingly large. It is domestic violence, in particular, spouse abuse. In another time, wife beating was virtually ignored by law enforcement authorities, viewed as a private matter that families were free to settle by themselves. Police seldom arrested a wife-beater, and often an abused spouse was too intimidated or embarrassed to press charges. But a recent report of the National Institute of Justice reveals the shocking figures on the extent of this crime. Two million Americans a year are beaten by their spouses. More than 1.7 million are assaulted with guns or knives. No less than 8.4 percent of all the nation's homicides in 1984 involved one spouse killing another.

Some of these murders might have been prevented if police intervention had been more aggressive at earlier stages of trouble. But until a few years ago, it was thought that an arrest would be counterproductive -- that a violent husband would be likely to retaliate against his wife and children. Three years ago, though, a study undertaken by the Police Foundation demonstrated statistically just the opposite: men who were arrested after assaulting their spouses were least likely to commit the offense again.

Since that study, which was completed in Minneapolis, a number of urban police departments have begun training programs for officers that stress the deterrent effect of arrest as an alternative to the usual responses designed to minimize conflict and promote reconciliation. Seven states -- Connecticut, Delaware, Maine, Minnesota, North Carolina, Oregon and Utah -- have passed statutes requiring arrest in cases of probable domestic assault, even when the victim is unwilling to sign a complaint. In Connecticut, first-time misdemeanor offenders can be required to attend classes on the causes of rage and the means of controlling it, and if the violence does not recur, arrest records are expunged.

Domestic violence is present at all levels of society and is inflicted by spouses well known in their professions and communities. The certainty or even the probability of arrest is a powerful deterrent for this group of hidden offenders, and this response -- which would be obvious if the person committing the assault were a stranger -- is effective even in the family situation. The Minnesota study showed that an arrest breaks a cycle of violence and is more likely to promote real reconciliation in the long run than is passive acceptance or even mediation. It stands to reason that early intervention by police and courts will also bring down the discouraging rate of homicides-at-home. The seven states will be closely watched for results.