Mass Incarceration: Tell Hillary Clinton Punishment Matters

It's easy to forget that in 1991 the U.S. murder rate was well over twice what it is today. In a speech last week at Atlanta's Clark University, Hillary Clinton showed she doesn't understand her call for an "end the era of mass incarceration" will endanger lives. She now proposes to end mass incarceration by "keep(ing) more nonviolent drug offenders out of prison."

There are good reasons to decriminalize drug possession, though Clinton mentioned none. Furthermore, she is wrong in believing that decriminalization would end mass incarceration.

In 2012, less than 7% of inmates at state and federal prisons were in for possessing illegal drugs. And it was rarely just for possession of marijuana. There are no national data, but data from Arizona indicate as few as 0.3% were incarcerated for marijuana possession, and those cases involve people arrested multiple times.

In California, even adding together trafficking or possession offenses, only 1% of state prisoners are incarcerated for marijuana offenses.

To free a larger number of prisoners, you have to include all drug offenses, mainly trafficking. That would cut the prisoner population by 20%.

Clinton claims that putting people in prison for violating parole or minor drug offenses "does little to reduce the crime, and it tears apart families and communities."

But virtually no one is in prison for minor drug violations. And there are good reasons for requiring parolees to regularly report to a parole officer, keep their hands off drugs and firearms, and in some states take periodic polygraph exams. These rules help keep criminals from committing still more crimes.

Unfortunately, most criminals go on to commit new crimes. Within a year, over 40% of released prisoners are arrested. Within five years, 77% return to prison.

Does sending someone back to prison "tear apart families"? Sure, but so does sending criminals to prison in the first place. Of course, the victims have also had their families torn apart by murder, rape and theft.

Clinton's speech raised another reason to release prisoners: the cost of imprisonment. In May this year, she claimed she can cut the prison population by over 50%. She said, "We would not be less safe." In Atlanta on Friday, she upped the ante and was talking about cutting the prison population by up to 87%.

Perhaps Clinton doesn't realize most prisoners are incarcerated for offenses far more serious than marijuana possession. She claims there are no benefits from the harm deterred by prison.

Myriad academic studies, including my own, show that harsher punishments (fines, imprisonment and the death penalty) deter crime. The states with the largest increases in penalties experienced the sharpest drops in crime rates over the past few decades.

In the U.S., it isn't just a coincidence that violent crimes fell by more than 50% from 1991 to 2014 at the same time that the percent of the population in prison rose by 53%. Economists call it the Law of Demand: Just as lowering the cost of apples means people will buy more apples, making crime less risky for criminals means more crime will be committed.

If Clinton wants to reform the system, she might try eliminating mandatory-minimum sentences more broadly than she is considering, particularly those that punish people for how they commit a crime rather than for the harm that they do.

Police are by far the single most important factor in reducing crime, but punishment matters too. Let's keep the high violent crime rates from the early 1990s a distant memory.

• Lott is the president of the Crime Prevention Research Center and the author of "More Guns, Less Crime" (University of Chicago Press, 2010).