When crime rates drop, the number of people in prison should fall in step—simple enough, right? 

Not so fast.   

A closer look at the statistical relationship between crime and incarceration rates suggests there is more to the equation.

Consider Oklahoma: The state’s property crime and violent crime rates fell beginning in 1996 but incarceration rates steadily increased after that time period as shown by the chart below.

This trend has generally held true nationally as well.

This statistical disparity between crime and incarceration rates might suggest a weak link between incarceration and crime and lend credence to the notion that law enforcement officials are continuing to put more people behind bars every year despite falling crime rates. However, there are explanations for the apparent disconnect.

Using a simple example, Dr. Peter K. Enns, a criminal justice reform expert and  professor at Cornell University, explains why incarceration rates may continue to rise despite significant decreases in crime rates.  

“Suppose 100 crimes are committed in a given year and all 100 perpetrators are arrested, convicted, and sentenced to three years in prison (with no parole). Now, suppose in the next year the crime rate drops tenfold and only 10 crimes are committed, with all 10 perpetrators arrested, convicted, and sentenced to three years. Because 100 individuals sentenced last year are still behind bars for another two years, the 10 new conviction will bring the total up to 110. Even though crime went down dramatically, the prison population grew because more people entered prison than were released.”

In other words, the length of prison sentences has a major effect on incarceration rates.

In his book, "Incarceration Nation," Dr. Enns also notes that other factors can lead to an increase in prison admissions. For instance, offenders who violate parole may go back to prison even though no new crime was committed. According to a state report from the Oklahoma Justice Reform Task Force, “around one-quarter of prison admissions in FY2015 were for a violation of supervision, of which just over half (55 percent) did not include a new conviction.”

One can apply Dr. Enn’s example to Oklahoma’s change in prison admissions to account for the state’s significant increase in its incarceration rate during the 1990s. 

During the 1990s Oklahoma, along with the rest of the United States, experienced a surge in violent crimes. This resulted in prison admissions outpacing releases throughout the decade leading to a higher incarceration rate.

As crime rates fell at the turn of the century, the incarceration continued to increase, but at a much slower rate. The continued increase in incarceration was because admissions continued to outpace releases due to long prison sentences handed out the decade prior.

Changes in penal codes and sentencing have a significant effect on a state’s future prison population. Take Oklahoma’s recent changes to its drug laws. Reclassifying minor drug possession as a misdemeanor instead of a felony will affect Oklahoma’s current and future prison population, especially since this change was applied retroactively to current inmates. Earlier this year NPR reported this change could impact the sentencing of at least 2,000 inmates.

According to a 2015 report from the Oklahoma Department of Corrections, more than a quarter of Oklahoma’s current prison population were convicted of drug related crimes.  (It’s important to note that not all these drug related crimes were for simple possession; many of those convicted of simple possession may have plead down to simple drug possession crime to avoid harsher sentencing for drug distribution and manufacturing.)

It’s imperative for Oklahomans to understand the lag time inherent in the criminal justice system between policy changes and the state’s prison population. Effects from recent legislative changes may not immediately affect the underlying data. Other factors can make the effects difficult to detect. However, this should not discourage policymakers or the public from continuing to reform Oklahoma’s criminal justice system to safely reduce its nonviolent prison population and leave behind its title as the No. 1 incarcerator.