It seems to be cause and effect. The US incarceration rate increased was stable for most of the 20th century. It was about the same in 1972 as it was in 1930: https://upload.wikimedia.org/wikipedia/commons/thumb/4/48/U..... By 2005 it had increased by a factor of 5.

That trend, however, lagged a similar increase in violent crime. Violent crime increased by a factor of five from 1962 to 1992: http://www.newgeography.com/files/cox-crime-3.png.

The causation seems to run in the direction of more crime leading to more incarceration. Incarceration rates stayed stable in the 1960s and early 1970s. By the time incarceration started ticking up in the early 1970s, violent crime had already increased by 2.5x. By 1980, the incarceration rate was about 40% higher than in 1960. But violent crime had increased by a factor of 4. The increase in incarceration rate didn’t catch up to the increase in violent crime rate until the mid to late 1990s, when violent crime started edging down.

I’m extremely sympathetic to the idea that criminal justice reform is required now that crime rates have gone down dramatically. But we need to grapple with our own history and the nature of our society. We went through a very violent phase from 1960 to 2000. We don’t know why that happened. But it seems pretty clear that our incarceration rate was a lagging response to that phenomenon.