The New York Legislature has passed and sent to Governor Spitzer a bill intended to deal with sex offenders whose prison sentences have expired. The theory is that sexual predators are likely to repeat their crimes after they are released. When the governor signs the bill, New York will become the 20th state to authorize continued “civil confinement” of sex offenders.

There is substantial anecdotal and statistical evidence of recidivism by sex offenders. In the most recent widely publicized case, a Florida man, John Couey, was convicted of murder for kidnapping and raping a 9-year-old girl and then burying her alive. Couey had previously been convicted of molesting a child.

The New York bill’s original sponsor, Senator Volker of Depew, promoted the bill for 13 years. It passed the Senate annually, but was shelved by Assembly Democrats, led by the speaker, Sheldon Silver, who found it excessively harsh and a violation of the civil rights of individuals who had already paid their debt to society.

Governor Pataki placed great importance on the issue to show Republican voters that he was tough on sex crimes. Mr. Silver, no friend of Mr. Pataki, would do nothing to assist him, even if it meant giving up the legislative salary increase that had been offered as a carrot.

At the start of the Spitzer administration, the governor’s lawyers began negotiating with Senate and Assembly counsel. All sides had been told to work until they reached agreement. They succeeded.

Objections to earlier bills were overcome by provisions mandating psychological treatment for offenders, and due process in determining whether an inmate remained a risk to the public. There is little evidence that treatment modalities now available are effective or not. If offenders remain incarcerated, communities will feel safer because fewer sexual predators will be released.

High-profile murders and rapes of children in recent years have resulted in adoption of various state laws, such as Megan’s Law in New Jersey, where 8-year-old Megan Kanka was raped and murdered in 1994. Megan’s Law required public notification to a community of the arrival of released sexual offenders. Similar federal legislation was passed by Congress and signed by President Clinton in 1996.

Other crimes led to other new laws, frequently named after their victims. Kendra’s Law in New York was passed after Kendra Webdale was pushed to her death under a subway train in 1999 by a mentally ill person who had stopped taking his medication. The bill was promoted by Governor Pataki, adopted, and later renewed. It mandates involuntary outpatient mental health treatment. If the former inpatient does not take his medicine, he will be returned to confinement.

Although no one is certain how to treat sex offenders, doctors agree that at least the sickest of them should be kept confined for as long as possible. Unfortunately it is impossible accurately to predict their future behavior.

Sex crimes, especially when youngsters are victims, are considered so horrible that society demands strong measures to protect potential victims. The Constitution prohibits cruel and unusual punishment, so the only remedy currently available is incarcerating offenders and attempting treatment, doubtful as the prospects may be.

The New York bill also creates a new class of crime: sexually motivated felony. Suppose a criminal enters a house at night, intending to rape a sleeping woman. He is caught before he can attack her. Today, he could be prosecuted only for breaking and entering. But under the new bill, since he came for sex, not just to steal, if that can be proven, he would be considered a sex offender. A state Office of Sex Offender Management would be created to oversee these programs.

Most legislation trades off costs and benefits. How does one measure the competing values here?

Opponents of the new state legislation argued that it provided insufficient protection for prior sex offenders, who could be imprisoned long after their sexual desires had disappeared. They rationally doubt the efficacy of psychiatric treatment, arguing that no psychiatrists would pronounce anyone cured for fear that they would be held responsible if the offender struck again.

The roll call on the bill was interesting, because both houses divided in an unusual manner. Majorities in both parties supported it, while what might be called the left wing of the Democratic Party opposed it. The legislation passed the Senate 51-8, and the Assembly 103-19. We will learn in time whether it works.

Mr. Stern, president of New York Civic, was New York City’s parks commissioner under Mayors Koch and Giuliani.