As state corrections budgets have shrunk in the wake of the Great Recession, so have their prison populations. Because U.S. prison systems have been bloated, inefficient and wasteful, the decline in the imprisonment rate — more than 10 percent over the last decade — is a step in the right direction. But while limiting the use of prison may cost less money, it will not magically lead to less crime. There’s a better way to downsize prisons, retain the cost reductions and still improve recidivism outcomes.

Increased participation by prisoners in effective programs can reduce recidivism. A large body of research, known as the “what works” literature, has shown that recidivism outcomes for prisoners are much better when they participate in interventions that target known risk factors for reoffending. Effective interventions include substance abuse treatment, cognitive-behavioral therapy, sex offender treatment, and education and employment programs.

But with five-year rearrest rates for released prisoners near 80 percent, recidivism outcomes have not been very good. A big reason is that many prisoners are idle in prison, due to their own choice or a lack of programming resources. When prisoners are “warehoused,” it diminishes their chances for success in landing a job and desisting from crime after they get released. A recent study found that warehousing increased the likelihood of recidivism by 13 percent.

But given that programming costs money, and a lack of money in state corrections budgets has been a key reason for downsizing, how can more programming be provided without increasing costs? The answer lies in further reducing the use of prison, which would yield decarceration “savings” that could then be reinvested to ramp up the delivery of effective programming. Decreasing the size of prison populations is necessary to not only reduce costs but also to free up the physical space needed within prisons — many of which are overcrowded — to provide more interventions.

There are two main ways in which prison populations can be reduced:

(1) Decreasing the number of people re-entering prison and (2) shortening the lengths of stay for those admitted to prison. To ensure that downsizing also produces better recidivism outcomes, determining what the length of stay should be is a key consideration. When individuals enter prison, it should be long enough to participate in effective programming, which usually lasts between three and nine months.

The best way to reduce prison admissions safely would be to restrict probation and parole violators (about two-thirds of all prison admissions) to the more serious offenders who are, as it is, more likely to get longer revocations. The less serious violators, who are more likely to get warehoused due to their relatively brief sentences, should remain in the community.

A reduction in probation and parole violator admissions will result in decreased imprisonment costs. These “savings” should then be reallocated to provide more programming resources for all probation and parole violators — those revoked to prison as well as those who stay in the community.

But if it is necessary to extend the minimum length of stay in prison to at least five months for rehabilitation purposes, the same holds true for limiting how long most inmates should be imprisoned. The average sentence length for prisoners is five years, which is ample time to participate in multiple effective interventions.

While lengthy periods of imprisonment may prompt some prisoners to reflect on their crimes and find remorse, the truth is that inmates with long sentences are likely to be warehoused for much of their confinement. For inmates with longer sentences who have participated in multiple effective interventions, trimming their confinement periods would generate decarceration “savings” that, once again, should be reinvested to increase the delivery of prison programming.

The elimination of the costly and, ultimately, ineffective practice of warehousing prisoners would constitute a big change in both our ideology and practice. After all, one enduring school of thought has been that if prison is so horrible, it will assuredly motivate inmates to quit crime. Increasing the misery of the prison experience may satisfy the impulse for retribution, but it doesn’t lead to an efficient use of taxpayer dollars. If we want prisons to be leaner, cost-effective and successful in reducing recidivism, we need reform based on what’s been shown to work.