It is no longer news that the United States incarcerates a higher proportion of its citizens than any other nation: more than 730 per 100,000, compared with less than 150 per 100,000 in Canada, Western Europe and China.

There are now more than 2.2 million inmates in American jails and prisons. Even this figure, however, undercounts the true size of the incarcerated population because it only includes those awaiting trial and those serving criminal sentences after conviction.

Here in America — alone among the nations of the world — we also incarcerate a growing population of ex-offenders who have done their time but remain behind bars because the government fears they might commit other crimes.

Since 1990, nearly 6,000 men have been designated sexually violent predators (SVPs) and confined to "secure facilities" under laws in 20 states — including New Jersey — and by the federal government.

Some of them have a history of violence, including rape. Others were convicted of crimes involving no physical contact, such as downloading child pornography.

Although their criminal histories vary, these men have two things in common: they have completed their sentences, yet remain indefinitely incarcerated under laws that permit the government to keep them locked up until (in the words of the New Jersey statute) they are no longer "likely to engage in acts of sexual violence."

For most of the men committed as SVPs, this amounts to a life sentence. From August 1999, when the New Jersey Sexually Violent Predator Act became law, through June 2012, 525 men were committed to the Special Treatment Unit (STU). (Don’t let the name fool you: the unit occupies a portion of the East Jersey State Prison, and comes complete with cells, tiers, concertina wire and daily lockdowns.)

During that period, only 47 men were released, most under intense supervision. Another 37 died in the STU or were sent to nursing homes to die. The outlook is even worse elsewhere. In Minnesota, almost 700 former sex offenders have been committed to high-security SVP facilities after completing their sentences. To date, only one has been released.

As a result of these trends, the STU costs more than $40 million to run in 2012, up from the $9.6 million it was projected to cost 14 years ago.

Nobody has a soft spot for sex offenders. No lobbyists protect their interests; no community groups fight for their rights. But the SVP laws undermine two principles of our criminal justice system: that we imprison people because they have broken the law — not because they might break the law in the future — and that once a convict has paid his debt to society, he may rejoin it.

These principles still apply to murderers, drug dealers and financial fraudsters. The difference in treatment, moreover, has nothing to do with re-offense rates. Data released by the Department of Justice in 2002 showed that 41 percent of drug dealers were re-arrested for a similar crime within three years after release from prison.

During the same period, less than 3 percent of released rapists were charged with a new rape. (The study followed prisoners released in 1994, before New Jersey and most other states enacted their SVP laws.)

Notwithstanding their relatively low recidivism rates, sex offenders are demonized in the media and singled out for exceptionally harsh treatment by lawmakers. An ever-growing list of "collateral consequences" ensures that their punishment will continue long after they have completed their prison sentences.

These consequences include offender registries, restrictions on where they may live and work, and — now in 20 states — SVP laws that allow the government to keep a growing number behind bars indefinitely.

Most experts agree that none of these measures does much to protect the public — in part because most sex offenses are committed by individuals with no previous sex convictions. But the measures remain popular among voters, and lawmakers have a hard time, politically, voting against new sex offender restrictions or easing existing ones.

We cherish our liberties. But we also, and all too easily, tolerate large-scale deprivations of liberty in the name of public safety, often without asking whether the price buys additional safety.

We made this mistake during World War II when we interned thousands of Japanese-Americans based on fear. We are making a similar mistake now. Worse yet, our legislatures and courts are once again growing comfortable with the notion of incarcerating an unpopular group because some members might commit crimes in the future. Before we proceed further down this slippery slope, we should rethink our nation’s SVP laws. •