It has become a boogeyman in public discourse: “mass incarceration.” Both left and right, from Hillary Clinton to Rand Paul, agree that it must be ended. But a close examination of the data shows that U.S. imprisonment has been driven largely by violent crime—and thus significantly reducing incarceration may be impossible.

Less than one-half of 1% of the U.S. population is incarcerated, according to the Bureau of Justice Statistics (BJS), so “mass” is a bit of hyperbole. The proportion of African-Americans in prison, 1.2%, is high compared with whites (0.25%), but not in absolute terms.

There’s a lot of historical amnesia about the cause of prison expansion, a mistaken sense that it was all about drugs or race and had very little to do with serious crime. This ignores the facts. Between 1960 and 1990, the rate of violent crime in the U.S. surged by over 350%, according to FBI data, the biggest sustained buildup in the country’s history.

One major reason was that as crime rose the criminal-justice system caved. Prison commitments fell, as did time served per conviction. For every 1,000 arrests for serious crimes in 1970, 170 defendants went to prison, compared with 261 defendants five years earlier. Murderers released in 1960 had served a median 4.3 years, which wasn’t long to begin with. By 1970 that figure had dropped to 3.5 years.

Unquestionably, in the last decades of the 20th century more defendants than ever were sentenced to prison. But this was a direct result of changes in policy to cope with the escalation in violent crime. In the 1980s, after well over a decade of soaring crime, state incarceration rates jumped 107%.

When crime began to drop in the mid-1990s, so did the rise in incarceration rates. From 2000 to 2010, they increased a negligible 0.65%, and since 2005 they have been declining steadily, except for a slight uptick in 2013. The estimated 1.5 million prisoners at year-end 2014 is the smallest total prison population in the U.S. since 2005.

Those who talk of “mass incarceration” often blame the stiff drug sentences enacted during the crack-cocaine era, the late 1980s and early ’90s. But what pushed up incarceration rates, beginning in the mid-1970s, was primarily violent crime, not drug offenses.

The percentage of state prisoners in for drug violations peaked at only 22% in 1990. Further, drug convictions “explain only about 20% of prison growth since 1980,” according to a 2012 article by Fordham law professor John Pfaff, published in the Harvard Journal on Legislation….

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