The three-decade-plus growth of incarceration in the U.S. at one point seemed an unstoppable public policy juggernaut, insensitive to political and economic changes. Until it wasn’t: The number of people in prison finally began dropping under President Obama and appears likely to continue doing so.

One key determinant of the size of the prison population is the number of admissions each year. I have presented a longer series of data on the prison admission rate before; here I will just focus on the past decade of data to better highlight something unusual about recent history. In interpreting the data, note that the government defines admissions as sentences exceeding one year, so every admission represents a unique individual.

The shape of the curve is singular. Initially the rate continues its decades-long ascent. But in 2006 it hits an invisible ceiling and begins plummeting with increasing speed. This is an unusual finding in public policy analysis. Particularly at the national level, it usually takes awhile for major policy changes to be consolidated. But in this case, we have experienced an unambiguous U-turn. Further, while the 2007 and 2008 drops in the rate of prison admission are roughly equal in size, from that point forward the drop each year exceeds that of the prior year. The drop in 2012 was about double that of 2010, four times that of 2009 and six times that of 2008.

I could tell a story about the causes of this policy turnabout and I am sure many other people could also (get the lead out, Kevin…). But all I want to do here is highlight the striking nature of the change: Outside of tulip bulb investing and the like, you rarely see national policy go so vigorously in one direction and then abruptly travel with accelerating speed in the opposite direction.

p.s. Some people have cited my prior posts on the prison admission rate change as “Humphreys’ research”, so let me clarify that I do not deserve credit for doing anything groundbreaking here. I am simply taking two numbers produced annually by your federal government (the number of prison admissions and the size of the general population) and dividing one by the other. In the chart above I multiplied the result by 100,000 as this is the standard incarceration rate used by criminologists.

[Cross-posted at The Reality-Based Community]

Keith Humphreys

Keith Humphreys is the Esther Ting Memorial Professor at Stanford University. @KeithNHumphreys