What the historical record does show is that many federal and state changes in criminal justice policy led to a fourfold rise in the incarceration rate from the early 1970s until it declined modestly in the last few years.

The rise in incarceration was driven by state laws like the 1973 Rockefeller drug laws in New York. And it was stoked by a major 1986 federal drug act, which expanded mandatory sentences and set the now-notorious 100-to-one ratio in the quantities of powdered versus crack cocaine that could trigger severe penalties.

Still, the incentives offered to the states in the 1994 law — nearly $10 billion for prison construction on the condition that states adopt “truth in sentencing” policies — may have added somewhat to the prison populations of the 28 states that took advantage of the provision. There, the results may still be playing out decades later as prisons are forced to establish geriatric units for inmates they cannot parole.

Incarceration’s disproportionate effect on African-Americans continues: In 2014, the Justice Department reported, 6 percent of all black men age 30 to 39 were in prison; the rate was 2 percent for Hispanic men and 1 percent for white men.

The steady rise in incarceration into the 21st century calls for further explanation because it persisted through a steep decline in violent crime.

No one could have known for sure in 1994, but the violent crime rate had already peaked. It plummeted over the next quarter-century, even as more people were sent to prison.