"U.S. Prison Population Trends 1999-2014: Broad Variation Among States in Recent Years"

The title of this post is the title of this notable short "fact sheet" from The Sentencing Project.  Here is the text that goes with the graphical state-by-state data in the document

The number of people in prison in the United States has stabilized in recent years, but incarceration trends among the states have varied significantly. While 39 states have experienced a decline since reaching their peak prison populations within the past 15 years, in most states this reduction has been relatively modest.  In addition, 11 states have had continuing rises in imprisonment.

Twelve states have produced double-digit declines within this period.  Four states have reduced their prison populations by over 20%: New Jersey (31% since 1999), New York (28% since 1999), Rhode Island (25% since 2008), and California (22% since 2006, though partly offset by increasing jail use).  Southern states including Mississippi and South Carolina, which have historically had high rates of incarceration, have also significantly downsized their prison populations.  These reductions have come about through a mix of changes in policy and practice designed to reduce admissions to prison and lengths of stay.  Moreover, the states with the most substantial reductions have had no adverse effect on public safety.

The overall pace of change, though, is quite modest given the scale of incarceration.  The total U.S. prison population declined by 2.9% since its peak in 2009.  Of those states with declining prison populations, 20 have had less than a 5% decline since their peak years.  The reduction in the federal prison population has been of this magnitude as well, 2.9% since 2011.  And of the states with rising prison populations, four have experienced double-digit increases in the last five years, led by Nebraska (22%) and Arkansas (18%). While sharing in the national crime drop, these states have resisted the trend toward decarceration.

Just as mass incarceration has developed primarily as a result of changes in policy, not crime rates, so too have declines reflected changes in both policy and practice.  These have included such measures as drug policy sentencing reforms, reduced admissions of technical parole violators to prison, and diversion options for persons convicted of lower-level property and drug crimes.