Prisoners rarely get released on parole, even when they’re no longer a threat. Here’s why.

Most states have the ability to release prisoners on parole after they almost surely pose no threat to the public. But a new report by the Marshall Project’s Beth Schwartzapfel reveals that states almost never do — and politics may be the reason.

This trend by states’ parole boards has fed into America’s mass incarceration problem, helping make the US the world’s leader in incarceration and helping sustain a bloated, costly prison system. And with advocates on all parts of the political spectrum looking at parole as one of many alternatives to longer prison sentences, Schwartzapfel’s report highlights how far the system has to come before it can really fulfill that goal.

When prisons release inmates before they complete their full sentence and keep them under government watch, it’s called parole. Most states allow parole boards to grant the early release, although many also limit or abolish parole altogether, as the map above by the Marshall Project demonstrates.

Parole boards, which are often made up of political appointees with no professional experience in criminal justice, evaluate whether someone should be released by looking at several factors — most importantly, crime severity, crime type, and criminal history. The hope is that these characteristics can give an idea about whether a prisoner will get out and commit more crime.

There isn’t any good national data as to how often parole boards grant parole, but Schwartzapfel’s investigation concluded that it’s very rare:

A months-long Marshall Project investigation reveals that, in many states, parole boards are so deeply cautious about releasing prisoners who could come back to haunt them that they release only a small fraction of those eligible — and almost none who have committed violent offenses, even those who pose little danger and whom a judge clearly intended to go free.

A recent revision of the Model Penal Code, an influential document written by legal scholars, declared parole boards “failed institutions.”

“No one has documented an example in contemporary practice, or from any historical era, of a parole-release system that has performed reasonably well in discharging its goals,” a draft of the document says.

When parole applicants are denied, they very rarely know the full reasons why. For example, most states won’t allow inmates to see or hear victims’ testimonies — so they can’t defend themselves from potentially false accusations during their parole hearings.

Very often, parole boards may know that prisoners, especially older inmates, aren’t really a threat to the public. (The research shows people tend to age out of crime.) But they won’t let them out anyway, out of fear of the political repercussions, Schwartzapfel explained:

Prisoners like Rodriguez represent a paradox for parole boards: Older inmates who have committed the most serious crimes, and served the longest terms, are the least likely to commit new crimes upon release.

One Stanford University study of 860 murderers paroled in California found only five returned to prison for new felonies, and none for murder.

This is especially true for older prisoners. Recidivism rates drop steadily with age. And older prisoners are more expensive: The average annual cost per prisoner doubles at age 55 and continues to climb thereafter.

Still, these prisoners are consistently the least likely to be paroled. Though they pose a low risk of future violence, the political risk of releasing them is huge. Parole board members are routinely pilloried in the news media and chastised by the public. Many have lost their jobs for releasing people whose crimes were violent.

Parole board members are most commonly appointed by the state’s governor. A board seat is typically a well-paid position with great benefits, Schwartzapfel reported. But if anything goes wrong, risk-averse governors and politicians will force board members to resign.

In Massachusetts, a man the parole board released went on to kill a cop during an armed robbery in 2010. The media, public, and police reacted with horror. Fox News wrote, “Massachusetts Cop Was Killed by Career Criminal Out on Parole Despite Three Life Sentences.” Gov. Deval Patrick (D) stated, “The public has lost confidence in parole, and I have lost confidence in parole.” The governor asked the parole board members involved in the decision to resign — even though he allegedly said that the parole board did nothing wrong.

Not only do parole boards face this kind of political pressure, but they often do so under huge time constraints. A 2008 parole board survey found the average state board considered about 35 decisions per workday in 2006, and these boards often have other responsibilities, according to Schwartzapfel.

“I typically voted 100 cases a day. That was just an average day,” an anonymous former Georgia parole board member told Schwartzapfel. “You’re just talking about two to three minutes to make a decision. The public would be astounded at the short period of time that the board has to make decisions on life and death cases.”

The result is a system that many criminal justice reformers call a failure. And with lawmakers from both political parties looking to shrink America’s massive incarcerated population, advocates like the Equal Justice Initiative want parole reform at the front.

Read the full report by the Marshall Project’s Beth Schwartzapfel.