Daniel S. Nagin is a professor of public policy and statistics at Carnegie Mellon University’s Heinz College of Information Systems and Public Policy. His research focuses on the evolution of criminal and antisocial behaviors over the life course, the deterrent effect of criminal and noncriminal penalties on illegal behaviors, and the development of statistical methods for analyzing longitudinal data.

In his State of the Union address, President Trump praised the First Step Act, a set of reforms aimed in part at winding down mass incarceration and reducing recidivism. Aptly named, the legislation is only a first step toward criminal justice reform, and a modest one at that. It affects only the federal prison system, which accounts for less than 15 percent of the nation’s total prison population, and even within the federal system, First Step’s impact on the prison population will be limited.

The imprisonment rate in the United States is now five times larger than it was in the early 1970s, and most of that increase happened at the state level. Marc Mauer and Ashley Nellis of the Sentencing Project have made a bold recommendation for unraveling mass incarceration — abolition of life sentences. Most lifers are in state prisons.

Research demonstrates that increases in already long prison sentences, say from 20 years to life, do not have material deterrent effects on crime. There is no good reason for believing that life sentences are a better deterrent than the Mauer-Nellis recommendation of a maximum sentence of 20 years.

The political and social causes for mass incarceration are complex, but the mechanism is easily described — the system sends more people to prison for longer periods of time. One unintended consequence of this is that our prisons have become old-age homes. Between 1993 and 2016, the percentage of U.S. prisoners ages 50 or older grew from 5 percent to 20 percent, and the number of those ages 40 years or older more than doubled, from 17.9 percent to 40.4 percent.

From a public safety perspective, this makes no sense. Decades of research by criminologists demonstrate that nature’s best cure for crime is aging — crime is a young man’s game.

The principal driver of the graying prison population is the growing proportion of lifers, mostly in state prison systems. One in 7 U.S. prisoners is now serving life or a virtual life sentence, a total of more than 200,000 people. In 1984, there were only about 34,000 lifers.

There has been a small decline in the nationwide prison population since 2009, but there exists broad consensus that much larger reductions are desirable. However, there is no consensus for how to do this without jeopardizing public safety.

There have been calls for incarcerating fewer nonviolent drug offenders, for example, but that wouldn’t do the trick. Drug offenders account for only 15 percent of the total prison population, and some are very dangerous. Rethinking life sentences, on the other hand, will do much more to reduce prison populations and also disproportionately reduce corrections budgets — the incarceration of infirm, elderly inmates is far more costly than their more healthy, younger counterparts.

The Mauer and Nellis proposal for complete abolition of life sentences is probably a bridge too far for our elected state legislators and governors. But more moderate changes, such as reducing the use of life sentences and increasing the possibility of eventual parole for those serving life, could have a significant effect without jeopardizing public safety.

Consensus on such action is possible. The passage of First Step is evidence that the need to reduce prison populations is one of the few pressing policy issues facing our country about which Democrats and Republicans agree. Reforming the life sentence should be the next step.