In 2015, 1.53 million people languished in US prisons and 728,000 in jails, or 673 per 100,000 (BJS 2016a, Table 1). Only North Korea, among major nations, may surpass the US in this regard.
Higher incarceration rates and longer sentences, along with the “war on drugs,” have imposed great costs on taxpayers, as well as on inmates, their families, and their communities. Yet even though the 59% per-capita rise in incarceration between 1990 and 2010 accompanied a 42% drop in FBI-tracked “index crimes,” researchers agree that putting more people behind bars added modestly, at most, to the fall in crime.
As for the implications for the impacts of mass incarceration on crime: Longer sentences do not clearly deter crime. THE IMPACTS OF INCARCERATION ON CRIME at page 130.
That is why states are finding new ways to get smart on crime and, in the process, changing how America views crime and punishment. After decades of rising prison populations, reforms in 33 states have helped cut the national incarceration rate by 13 percent since 2007.
SOURCE: THE IMPACTS OF INCARCERATION ON CRIME, Open Philanthropy Project, September, 2017, https://blog.givewell.org/wp-content/uploads/2017/09/The-impacts-of-incarceration-on-crime-10.pdf
LESS INCARCERATION, LESS CRIME, The PEW Charitable Trusts, March 21, 2017, http://www.pewtrusts.org/en/multimedia/audio/2017/less-incarceration-less-crime