If a convicted child molester moved into the house across the street, I'd want to know. But I'd also want to know if my new neighbor had been convicted of homicide, assault, robbery, burglary, larceny, or fraud.

Which suggests one of the problems with sex offender registration laws, the focus of two cases the U.S. Supreme Court is scheduled to hear on Wednesday. These laws—requiring sex offenders who have served their sentences to report their whereabouts to the government, which passes the information on to the public—are both too narrow and too broad.

They are too narrow because they do not cover a wide range of potentially dangerous characters whom citizens might want to avoid. They are too broad because the sex offender label sweeps together serial predators with individuals who pose little or no threat to the public. For example, Alaska's law, one of the two the Supreme Court will consider, applies to people convicted of possessing child pornography.

The main rationale for singling out sex offenders is the assumption that they are especially likely to commit new crimes. Sex offenders "will immediately commit this crime again at least 90 percent of the time," a California legislator warned in 1996.

The Bush administration—which filed a brief in defense of Connecticut's registration law, the other statute the Supreme Court will consider—is a bit more cautious. "When they reenter society at large," says Solicitor General Theodore Olson, "convicted sex offenders have a much higher recidivism rate for their offense of conviction than any other type of violent felon."

The brief cites data from the Bureau of Justice Statistics, which show that rapists are more likely to be rearrested for rape than other offenders are. But that does not mean they are more likely to be rearrested.

Among prisoners released in 1994, 46 percent of rapists were arrested again for any offense within three years, compared to 62 percent of violent felons generally. Recidivism rates for nonviolent criminals were even higher: 79 percent for car thieves, 74 percent for burglars.

Even if we focus on repeats of the same offense, rapists do not stand out. Less than 3 percent of them were arrested for a new rape in the three years covered by the study. By comparison, 13 percent of robbers, 22 percent of (nonsexual) assaulters, and 23 percent of burglars were arrested again for crimes similar to the ones for which they had served time.

Studies that cover longer periods and include other kinds of sex offenders find higher recidivism rates, but still nothing like those claimed by politicians. The National Center on Institutions and Alternatives cites three large studies, covering tens of thousands of sex offenders, that reported rearrest rates for sex offenses ranging from 13 percent to 19 percent.

It seems that the vast majority of people forced to register as sex offenders are actually former sex offenders who will not repeat their crimes. Indeed, Connecticut's online Sex Offender Registry proclaimed that "the Department of Public Safety has not considered or assessed the specific risk of reoffense with regard to any individual prior to his or her inclusion within this registry, and has made no determination that any individual included in this registry is currently dangerous."

According to the U.S. Court of Appeals for the 2nd Circuit, that was precisely the problem. The court ruled that Connecticut's registration law violates the Due Process Clause because it does not give offenders an opportunity to challenge the presumption that they are public menaces.

In the Alaska case, the U.S. Court of Appeals for the 9th Circuit did not address the due process issue because it concluded that the statute was an ex post facto law, unconstitutionally imposing punishment on offenders who committed their crimes before it was passed. As evidence of the law's punitive effect, the court cited, among other things, onerous requirements similar to those of probation: Many offenders have to register in person with police four times a year for the rest of their lives.

The court also noted that "by posting the appellants' names, addresses, and employer addresses on the Internet, the Act subjects them to community obloquy and scorn that damage them personally and professionally," making it difficult for them to find work and lead normal lives. One need not have sympathy for sex offenders to wonder whether this is a sensible way to encourage their rehabilitation.