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In theory, imprisoning more people reduces crime in two ways: by incapacitation (those locked up cannot commit more offenses) and deterrence (individuals refrain from crime after seeing others suffer consequences of imprisonment). If incapacitation and deterrence theory are valid, we would expect increased incarceration to correspond to reduced crime rates among populations most affected by high rates of imprisonment.

If increased imprisonment reduced crime by incapacitation, felony trends generally should move in the opposite direction of imprisonment trends – as one rises, the other falls. Arrest rates are the best (though far from perfect) measure available of crime rates by age, and, arrest trends for felonies (the crime leading to imprisonment) closely track reported crime trends.


Figures 1 – 4 compare the felony arrest and imprisonment rates for 1980 through 2017 (DOJ, 2018; CDCR 2018).

The fundamental mistake analysts have made in analyzing crime-versus-imprisonment trends is the assumption that crime” is a unitary phenomenon across all demographic groups. That assumption produces a seeming pattern of generally increased imprisonment accompanying generally decreased crime (Figure 1), yielding the weak correlation by which some analysts have claimed that imprisoning more people in the 1990s reduced crime back then by around 10 to 25 percent (see Pew Trusts, 2015). Researchers also have used reported crime measures that include crime involving juveniles but have failed to incorporate juvenile incarceration trends.

Figure 1. Annual arrest and imprisonment rates in California per 100,000 population, 1980 – 2017 average, all ages

However, what we call crime” is not a unitary trend, but an amalgamation of large decreases among younger (under 25) ages, smaller decreases among young adults (2539), and increases among older adults (40 and older) (Figures 2 – 4).

Figure 2. Annual arrest and imprisonment rates in California per 100,000 population ages 10 – 29, 1980 – 2017

Figure 3. Annual arrest and imprisonment rates in California per 100,000 population ages 25 – 39, 1980 – 2017

Figure 4. Annual arrest and imprisonment rates in California per 100,000 population ages 40 – 69, 1980 – 2017

Sources: CDCR (2018); DOJ (2018). The linear trendlines are the product of the regression equation for each series that incorporates all data points.

When age groups are separated, it can be seen that greatly decreased crime rates among youth and young adults accompany long-term, decreased imprisonment of this younger population in prisons run by the California Department of Corrections and Rehabilitation (CDCR) and juvenile facilities run by the Division of Juvenile Justice (DJJ). Meanwhile, major increases in the imprisonment of older Californians (ages 40 and older) paralleled generally increased crime rates. Only for adults ages 25 – 39 does moderately increased imprisonment accompany modestly decreased crime, and this pattern appears to be an artifact (see Conclusion).


California’s criminal justice system has entered a new phase in which the demographics and characteristics of arrested individuals have changed dramatically, requiring innovative policies to address current needs. One of the most stunning failures of crime analysis is its adherence to 1990s assumptions that ignore 21st-century realities. From 1980 to 2017, felony arrests of persons under age 25 fell by two-thirds while felony arrests of those age 40 and older tripled. In 1980, 4.3 times more people under age 25 were in prison than 40 and older; in 2017, 4.3 times more age 40 and older were in prison than under 25. Since incapacitation by imprisonment varies sharply among different groups over time, it makes no sense to analyze crime trends” without accounting for major demographic shifts.

If Group A experiences increased imprisonment and Group B experiences reduced imprisonment, incapacitation theory holds that crime should fall in Group A compared to Group B. Exactly the opposite occurred in California. Decreased imprisonment of ages under 25 accompanied greatly decreased crime in that group, while greatly increased imprisonment of ages 40 and older accompanied increased crime. Only in the middle group, adults age 25 – 39, did increased imprisonment accompany decreased crime, both very modest. Based on the patterns of older and younger ages, it appears that trends among ages 25 – 39 result from the steady movement of under-25 ages with decreasing crime rates into this middle group.

Theories of incapacitation and deterrence are countered by evidence that decreased crime rates among youth and young adults are associated with decreased imprisonment, and increased crime rates among older adults accompany increased imprisonment. Excessive imprisonment can contribute to more crime, particularly today, when those involved in crime are more likely to suffer addictions that prevent sober weighing of costs and benefits. Finally, the costs of imprisonment reach beyond the individual, negatively impacting families and contributing to the challenges of safety and well-being within communities. Reducing imprisonment is a fiscally, constitutionally-mandated, and humanitarian imperative California is accomplishing without suffering more crime.