One after another, the laws keep coming.

Communities concerned about the access sex offenders have to children have passed ever-tighter legislation restricting where convicted child molesters, rapists and other sex offenders can live. Nationally, 21 states and hundreds of municipalities have similar laws on the books.

In Maine, three communities have enacted ordinances to prevent sex offenders from living near schools, day care or preschool centers, and even libraries or public parks. Lincoln passed such an ordinance on Dec. 11, joining the York County towns of Waterboro and Lyman.

But experts in sex-offender treatment and recidivism say there is little proof such measures keep communities safer or prevent sex offenders from striking again. In Iowa, for example, the number of registered sex offenders unaccounted for more than doubled after a strict residency law went into effect. And studies of supervised sex offenders in two other states indicated that where offenders lived had no impact on new sexual offenses they committed.

“There really isn’t any empirical evidence to say they are a viable strategy for keeping communities safe,” said Jill Levenson, a human services professor at Lynn University in Boca Raton, Fla., and a board member of the Association for the Treatment of Sexual Abusers.

Levenson and other experts say making it harder for sex offenders to find housing can lead to stress and instability, which can increase the likelihood they will re-offend.

Law enforcement officials caution that housing restrictions can result in clustering of sex offenders in certain areas. Long Island, N.Y., already has more than a dozen such clusters. The laws also can create more homeless or chronically transient sex offenders, making it harder to track them.

“We’re trying to separate them from vulnerable people,” said Joseph J. Abramo, the supervising probation officer for the Suffolk County, N.Y., sex offender unit. “But it does create stressors on them, and I hope it doesn’t cause them to go underground or act out.”

The Long Island laws are part of a national trend that began a few years ago and snowballed in the wake of high-profile crimes, such as the killing last year of 9-year-old Jessica Lunsford by a sex offender in Florida, who lived nearby.

Earlier this year, Suffolk County passed a law forbidding sex offenders from living within a quarter-mile of schools and playgrounds.

Soon after, neighboring Nassau County set the limit at 1,000 feet from schools and 500 feet from public parks.

Lawmakers behind such bills say they are common-sense edicts that place limits on sex offenders who aren’t supervised once off of probation or parole. While restrictions alone won’t solve the problem, Suffolk County legislator Jon Cooper said, “If we’re serious about this as a society, wanting to protect our kids, we need to put our money where our mouth is.”

But some question the assumptions that shape residency restrictions, which they call a one-size-fits-all approach to a complex problem.

“Not all offenders are the same, and not all offenders pose the same risk,” said Charles Onley, of the Center for Sex Offender Management in Maryland.

The laws in New York’s Nassau and Suffolk counties apply to all registered sex offenders, whether they have abused minors or adults. And regulations, such as the residency laws, aimed at preventing strangers from preying on children do not address the vast majority of sex offenses, which statistics show are committed by those knowing their victims.

“A lot of these offenders are people who have access to your children or to you,” said Onley, adding that overall, between 70 percent and 80 percent of sexual offenders, including repeat offenders, know their victims.

An FBI study found more than 92 percent of girls 17 and under and 95 percent of boys in the same age group reporting a sexual assault identified a family member or acquaintance as the culprit. “It’s date rape, a priest, your uncle or granddaddy, the schoolteacher,” Onley said. “It’s not the guy hiding behind the bush.”

A 2004 report on sex-offender housing by Colorado’s public safety department said distance markers from schools and parks may not deter recidivism.

A report the year earlier by Minnesota’s department of corrections tracked 329 of the state’s most serious sex offenders, knows as Level 3’s. It found that the location of the homes of those offenders relative to places where children congregate had no bearing on their subsequent sex crimes.

Thirteen of the 329 re-offended. Two of the 13 did so after driving from their homes to parks several miles away.

“Enhanced safety due to proximity restrictions may be a comfort factor for the general public, but it does not have any basis in fact,” the report said.

That danger is not lost on local law enforcement officials faced with enforcing new residency restrictions. “While it’s true they can’t live there, there is nothing to say they can’t be sitting in the park with a bunch of balloons and animals when the kids come,” said Nassau County probation director John Carway. That was echoed by Florida sex offenders Levenson surveyed in a 2005 study on their attitudes toward that state’s 1,000-foot proximity law. Most surveyed said the rule wouldn’t affect their risk of re-offense.

Long Island lawmakers who back residency limits often say high recidivism rates among sex offenders justify blanket restrictions. According to an oft-cited 1994 federal Bureau of Justice Statistics study, male sex offenders were four times as likely as non-sex offenders to commit a sex crime in the three years after their release from state prison.

“Most sex offenders do not re-offend,” said Karl Hanson, a senior research officer with Public Safety Canada who studied sex offenders for two decades.

On average, he said, sex offenders have a 10 percent to 15 percent recidivism rate five years after their release; that rate rises to about 20 percent after 10 years.

Those who support residential restrictions for sex offenders say the laws might not be perfect, but they’re an improvement over the lack of supervision in New York for offenders no longer on parole or probation. “It’s really a reaction to the lack of funding being allocated to monitor and supervise sex offenders for life,” said Laura Ahearn, of the Stony Brook, N.Y., advocacy group Parents for Megan’s Law.

But questions remain about the unintended consequences such laws can inflict.

“What we’re seeing on a national basis is that, the more restrictive the residency requirements become, the more frequently sex offenders fail to comply and become ‘whereabouts unknown’ and drop off the radar screen,” said Richard Hamill, head of the New York State Alliance of Sex Offender Service Providers.

In Iowa, the number of registered offenders with no known address more than doubled since a state law banning sex offenders from living within 2,000 feet of a school or child care facility went into effect last year.

Iowa’s law – one of the most stringent in the country – has undermined rehabilitation of sex offenders by making it nearly impossible for them to find housing, jobs or sustain a family life, according to a statement this year from the Iowa County Attorneys Association. The prosecutors’ group said Iowa’s residency restriction compromised the safety of children.

It is not clear what will happen on Long Island, where distances are smaller and restrictions are newly imposed. But those who treat sex offenders say the instability such rules can cause – from constant evictions to the inability to live with family members whose homes lie within buffer zones – can pose another threat to public safety.

“This is a population that doesn’t deal well with stress,” Hamill said. “When we create policies that cause them to lose their housing and lose their jobs, many don’t respond well. And for some, committing sex offenses is a way of managing that stress.”