Incapacitation Research Paper

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Criminologists typically consider four possible mechanisms by which incarceration can prevent crime: general deterrence, specific deterrence, rehabilitation, and incapacitation.

  • General deterrence focuses on the changes in behavior created by the threat of punishment. Individuals may choose not to do a particular crime in order to avoid the possibility of incarceration.
  • Specific deterrence focuses on the possible change in behavior after the experience of prison. An individual who has experienced prison may update their expectations about prison and decide, on the basis of that experience, to avoid criminal activity in the future. Of course, the experience of prison may also be less harsh than the person was expecting, leading to an increase in the rate of offending over what the person would have committed absent the prison experience.
  • Rehabilitation refers to changes in the underlying “criminogenic” factors that the individual brings to prison, usually through targeted treatment programs, like drug treatment, cognitive behavioral programming, or work pro- grams. These programs (and the resulting changes in the underlying propensity to offend) will lead to reduced crime by these individuals upon release. However, the prison experience itself may also increase criminal propensity, by reducing the person’s prosocial human capital or increasing the person’s criminal capital, such that the person is actually more likely to offend upon release.
  • Incapacitation reduces crime by literally preventing someone from committing crime through direct control during the incarceration experience. While it is not impossible to commit a crime in prison, the possibility is greatly limited by the direct control exerted by the correctional system. People who are in prison will not be able to commit crime at the same rate as they did prior to their prison experience, and these prevented crimes should be a direct benefit to society. Unlike the first three mechanisms, incapacitation does not rely on creating changes in criminal propensity. Criminal propensity does not change at all – it simply is prevented from becoming reality. This direct, obvious connection between incarceration and crime reduction is the main attraction of incapacitation. The main drawbacks are that there are no efficiencies to scale and the effect is time limited. While one prison cell could potentially deter the criminal actions of many people or rehabilitate a person for a lifetime, one prison cell can only incapacitate one criminal at a time for the time that the person is in prison.

The benefits of incapacitation depend directly on the offending behavior of those individuals who are incarcerated. Incapacitation benefits will be larger for policies that manage to incarcerate higher rate criminals. This is particularly true given that most criminologists believe that criminal offending is highly skewed among the offending population, with a relatively small minority of all offenders responsible for the majority of all crimes. Selective incapacitation focuses on the idea that policymakers can prospectively identify the most active offenders prior to their period of peak activity and prevent a great deal of crime through “selectively” incapacitating these high-risk individuals. However, incapacitation also implies declining marginal returns. If society starts by incapacitating the highest rate and most frequent offenders, additional incarceration will generate reduced benefits from incapacitation as society incarcerates lower rate, less frequent offenders.

Estimating Incapacitation

Historically, there are two basic approaches to the study of incapacitation that fall along basic disciplinary lines. The first, based in criminology’s criminal career literature, relies on self-reported data from incarcerated individuals to generate estimates of lambda, the frequency of offending. The second, based in economics, relies on aggregate data from places to estimate the impact of prison on crime rates.

Criminologists use individual estimates to generate simulated estimates of the amount of crime averted by specific imprisonment policies. For example, inmates have been asked while incarcerated to describe their offending prior to the current term of incarceration. These responses are then used to generate estimates of the annual amount of crime committed while the person is free using the criminal career approach. The criminal career approach comes from operations research and involves the use of detailed equations which identify the specific parameters that contribute to an observed offending rate. Estimates of the underlying parameters are then used to generate an estimate of the benefits of a year of incarceration in terms of the number of crimes prevented.

Initially, research using this approach reported extremely high benefits from incapacitation, on the order of almost 200 felony crimes per prisoner year. But data on self-reported crimes is highly skewed, with a few offenders reporting a great many crimes. More recent research has reached a consensus around 15–20 felony or index I crimes per year in prison, on average. However, there is no way of knowing whether a given policy change will incapacitate the average offender or someone who commits less crime on average.

There is also no way of knowing whether the person placed in prison was simply replaced by someone else who would have otherwise not committed crime. The possibility of replacement is most plausible in the case of drug crimes, where dealers could be replaced by others. The problem of replacement is similar to the problem of displacement in the case of place-based crime prevention, which attempts to reduce crime by making a “crime-generating” place incapable of generating crime (by, e.g., increasing police presence). If criminals simply go to another place, then crime is displaced, and overall crime is not reduced by this policy, even though crime goes down in a particular place. By the same token, crime is not reduced by incapacitation if a person is incarcerated and is promptly replaced by another individual who now commits the same crimes the other person would have committed absent incarceration.

One way around the problem of replacement is to focus directly on the total amount of crime committed in a place rather than on the crime committed (or not committed) by a particular person. A change in the number of crimes committed in a certain place that can be directly tied to changes in the number of people in prison will generate an estimate of the incapacitation effect that is net of replacement. This idea of linking changes in incarceration to changes in crime for places is the approach of economists. While this approach has the twin advantages of controlling for replacement while linking policy directly to the outcome of interest, it also faces numerous empirical challenges, most notably the problem of endogeneity between prison policy and crime rates.

There is a positive correlation between aggregate crime rates and incarceration rates, not because prison causes crime but because places with higher crime rates also tend to put more people in prison. Attempts to causally identify the impact of prison on crime must therefore break this link by identifying exogenous variation in the incarceration rate. Experiments can cause this variation, although it is hard to create experimental variation in incarceration.

However, “natural” experiments can sometimes create exogenous variation. For example, Steve Levitt observed that some states in the USA were forced by the courts to reduce their incarceration levels due to charges of overcrowding (Levitt 1996). Initially, at least, states under court sanction could only meet this mandate by releasing prisoners. Likewise, the Italian government routinely releases up to 35 % of its prison population through periodic “collective pardons” (Barbarino and Mastrobuonie 2012). This creates variation in the incarceration rate over time and in different places, because the pardons release varying amounts of prisoners to each Italian region. In each case, researchers have observed an immediate increase in crime rates in the places that receive prisoners relative to places that do not, a change that that can be plausibly associated with the individuals who were just released from prison who otherwise would have been incapacitated. These papers generate estimates of between 15 and 20 more felony crimes per year in prison, a number that aligns well with the individual estimates from criminology.

It may not be completely correct to attribute all of these changes to incapacitation. In each case, the deterrent threat of incarceration has also changed as has the potential for specific deterrence and rehabilitation. For example, an individual contemplating crime in a state with a court-imposed cap on incarceration might plausibly assume that his chance of incarceration is less if he was to commit a crime. Individuals who are part of collective pardons might not be able to complete rehabilitation programs or might decide that incarceration is not as bad as they had previously thought and therefore commit more crimes than they would have without the collective pardon.

Some researchers have attempted to differentiate between incapacitative effects and deterrent effects by looking at the timing of effects on crime (Kessler and Levitt 1999). For example, three strikes laws impose long prison sentences for multiple or repeat offenders. These individuals would have been incarcerated even without the three strike provisions: the difference is that they may now have a 10or 15-year sentence instead of a 5-year sentence. In the short term, there will be no additional incapacitative benefit to these particular laws. An immediate change in the crime rates after the implementation of the law can then plausibly considered to be a deterrent rather than an incapacitative effect. Long-term effects can be plausibly attributed to both deterrence and incapacitation.

Miles and Ludwig (2007) argued that reliable and valid estimates of incapacitation are too difficult to obtain and that time would be better spent generating estimates of the aggregate effects of prison using natural experiments. Although these aggregate measures would not enable the researcher to isolate the mechanism by which crime declined, it would give policymakers clear guidance with which to conduct cost-benefit analyses of prison. In a cost-benefit analyses, the cost of incarceration is compared with the monetized benefits of preventing a certain number of crimes. The monetary costs of crime are generated using direct accounting methods, compensatory damages from civil lawsuits, and methods that attempt to put an estimate on individuals’ willingness to pay to avoid victimization. From Miles and Ludwig’s perspective, generating a clean estimate of the crime-reducing benefits of a policy would then provide room to concentrate on generating good estimates of the costs and benefits of such a policy.

Recent work by economist Ben Vollaard (2012) looking at the “habitual offender” policy in the Netherlands is an exemplar of this tradition. He first estimates the reduction in crime due to the policy and then conducts a cost-benefit analysis to determine its viability as a crime prevention strategy. While Vollaard argues that the observed impact is due to incapacitation of chronic drug-addicted offenders who were no longer “deterrable” or amenable to treatment, he also acknowledges that it is still at least possible that there was also deterrent value to the statute. For example, good evaluations of “three strikes” laws in California using individual-level data have plausibly identified deterrence for individuals exposed to the risk of the third strike.

Regardless of whether or not the effects identified in the paper are entirely the result of incapacitation, the paper is important because it simultaneously highlights the attraction and limitations of incapacitation policies. Vollaard’s estimates are huge, suggesting that the policy prevents over 50 reported crimes per prison year. This very large effect size is actually plausible because of the policy’s laser focus on very high-rate habitual offenders in a country with a very low incarceration rate. The average offender sentenced under the law had 31 prior felony convictions, almost all for property offenses. The policy is implemented only as a last resort, after the person has shown no response to treatment. This makes sense, because in the long run it will be cheaper to simply remove the “cause” of the crime problem (the drug habit). Incapacitation only works while the person is in prison, while rehabilitation could last “forever.” In cases where treatment does not work, Vollaard’s paper appears to show that incapacitation could be a very cost-effective policy for reducing crime.

The problem arises when policymakers get greedy. If we can stop 50 crimes by incarcerating someone for a year, why do we not incarcerate more people? The reality is that only a few people commit crime at such a high rate. In the Netherlands, a country of 16 million, only 4,000 people are even eligible for the habitual offender label in a given year, and even fewer actually receive the penalty. Nevertheless, Vollaard shows that Dutch cities that used the habitual offender law more liberally also had smaller crime reduction per prison year. The reality of a strong positive skew is impossible to avoid – the only way to incarcerate more people is to incarcerate offenders who commit fewer crimes. This leads inevitably to diminishing marginal returns from increased incarceration. In the USA, which has seen a fourfold increase in the incarceration rate over the last 40 years, researcher after researcher has shown that the impact of incarceration on crime has declined during this period (e.g., Piehl et al. 2006). Incapacitation may work, but it is not scalable without a loss of impact.

Selective Incapacitation

The second way policymakers may get greedy would be to avoid waiting until someone reached “habitual offender” status. In the Netherlands, the habitual offender statute could only be used as an option of last resort for an offender with at least 10 felony offenses. As a result, individuals are only incapacitated after they have revealed through their own behavior that they are indeed prolific. However, it is tempting to avoid waiting until someone has committed so many offenses before identifying them as a high-rate offender. Imagine how many crimes could have been prevented in the Netherlands if these individuals could have been identified and incapacitated before they were convicted more than 10 times for felony offenses.

Prospectively identifying high-rate offenders, sometimes called selective incapacitation, holds out this tantalizing prospect of dramatic reductions in crime. It also carries with it some ethical ambiguity, since individuals are essentially being incarcerated for crimes they have not yet committed, and the inevitable cost of false positives. False positives are people identified as high-rate offenders who would have stopped without any additional intervention. Because of this concern about false positives, the debate about selective incapacitation can become quite heated, and people often make broad statements questioning whether criminologists can reliably predict high-rate offenders.

A reasonable read of the now large literature on identifying chronic offending and recidivism will necessarily conclude that criminologists can prospectively identify high-rate offenders at a rate significantly (and substantially) better than chance. It is also true that the accuracy is far from perfect. The open question is not whether criminologists can predict risk – they can – but whether the accuracy is good enough to create benefits that outweigh not only the costs of incarcerating the accurately identified high-risk offenders but also the costs generated by incarcerating the false positives.

There may also be inherent limits to the ability of researchers to prospectively identify high-risk offenders in the current environment. Empirical models require data on individuals followed over many years to validate risk prediction models. This is not a big problem in places like the Netherlands, since even the prolific offenders in the Netherlands who were subject to the habitual offender law had spent very little time in prison prior to receiving the sentence enhancement. However, in the USA, these people would have been incarcerated for substantial periods of time, drastically reducing the amount of time in which their behavior could have been observed. As a result, it would have taken them longer to accumulate the same number of offenses.

Despite the presence of these challenges, the use of risk prediction in the USA’s criminal justice system, for sentencing, correctional placement, probation supervision, and parole release, has exploded and shows no sign of abating. Although not all risk prediction is used for selective incapacitation, many of the explicit goals of the risk, needs, and responsivity (RNR) model that now dominates the risk prediction field are at least consistent with selective incapacitation. For example, a central tenet of the RNR approach is the identification of low-risk offenders who will not offend even without treatment or supervision. This practice is simply selective incapacitation at the other end of the distribution – why incarcerate or otherwise restrict people who are at low risk for offending.

The Future Of Research On Incapacitation

For decision makers, the most important question is the extent to which incarceration reduces crime; the path generating the reduction is irrelevant for those purposes. However, incapacitation has acquired prominence primarily because of its “obviousness.” It is easy to question the power of deterrence in a world where people shoot one another over Nike sneakers. It is not easy to question that locking someone up will prevent the crimes he would have committed had he not been locked up. The debate, therefore, is not about the existence of the impact, but the relative costs and benefits of the approach. Within that framework, plausible estimates of the size of incapacitation benefits serve as useful lower bounds on the potential benefits of incarceration.

As previously discussed, there have traditionally been two approaches, one which relies on inmate surveys and the criminal career approach and the other which relies on policy changes and aggregate crime rates. The first approach, which relies on complicated calculations and expensive surveys, has not been used regularly for at least 15 years, and the second approach, taken to its logical end, reaches the conclusion that the incapacitation effect cannot be isolated from all the other mechanisms by which prison reduces crime.

A third approach has emerged in the last 5 years that combines the best parts of the two methods. The main advantage of the first approach is that it uses data from individuals and allows researchers to estimate a distribution of benefits from incapacitation. The main advantage of the second approach is that it provides a useful counterfactual, comparing the crime control effects of two different policies that can be described substantively.

These new attempts to estimate the benefits of incapacitation rely on individual-level data, like the first method, but, like the second method, attempt to create a reasonable counterfactual. In this approach, individuals are identified who are incarcerated. The researcher then creates a useful counterfactual using other observations in the data. The rate of offending generated by the non-incarceration counterfactual becomes the best estimate of the amount of crime prevented through the incarceration of the first individual.

The main attraction of this latter approach is that it does not force the researchers to assume that individuals who are not incarcerated will necessarily look like those who are. Rather, individuals are explicitly identified as comparable on the basis of their observed offending behavior and other characteristics.

For example, criminologists Gary Sweeten and Robert Apel (2007) studied the self-reported offending of a group of individuals in a contemporary USA sample. In contrast to most prior research, they do not generate estimates by relying on the reports of offending before incarceration by the same respondents. Rather, they rely on self-reported data from a matched control group (using propensity scoring) of other people who are otherwise similar but not incarcerated. This is the first time that a matched sample approach has been used to generate estimates of incapacitation.

Since it is likely that those who are incarcerated are different from those who are not in unobservable ways, it is also reasonable to assume that the Apel and Sweeten estimates also underestimate the incapacitative benefits. Their approach also has the merit of being tied to a specific change in imprisonment policy, namely, increasing the numbers who are given prison as a punishment as opposed to extending the lengths of those currently incarcerated.

Economist Emily Owens (2009) fashioned an identification strategy that should control for both unobserved and observed differences between those who are incarcerated and those who are not. It also fits nicely between the two approaches, because she uses individual-level data and a natural experiment to estimate the causal effect of incapacitation for a subsample of people affected by the policy. She takes advantage of a technical change in Maryland sentencing guidelines that had a substantial effect on the sentence for a subset of sentenced offenders.

The change involved the use of juvenile records in sentencing decisions. Until 2001, these records were included in the criminal history of all individuals up to the age of 25; after 2001, the age for which juvenile histories counted was lowered to 22. Thus, some of those aged 23–25 received shorter sentences than they would have received in the earlier years. Owens estimates that this reduced the average sentence under the Maryland guideline system by about 25 % (about 9–18 months). She also found that, during this time period when they were at liberty because of the change in rules, youths sentenced after 2001 were arrested on average 2.5 times per year. Taking account of the specific offenses for which they were arrested and the ratio of recorded arrests to recorded offenses of the same type, she estimates that they were responsible for 1.5 index crimes per year. This provides a relatively precise estimate of their recorded criminal activity during a period when they would have been incarcerated under the previous rules.

Unlike Sweeten and Apel, Owens’ estimate uses information from other offenders to generate a counterfactual incarceration spell, rather than a counterfactual offending rate. Then, the individual’s own offending during this time period after their initial period of incarceration is used to estimate the incapacitation effect. The estimate that Owens develops of crimes averted is smaller by an order of magnitude than the consensus estimate of 15–20 crimes previously cited in the literature. The difference comes from the fact that Owens has in all likelihood captured the lower tail of the distribution by her focus on young, relatively minor offenders.

The causal modeling in Owens’ paper represents strong causal identification, but it only provides an estimate for a very narrow part of the population. This trade-off between generalizability and strong internal validity is probably typical of the research that might follow in this vein.

A final trend in this area is to extend the concept of incapacitation to include other policies besides incarceration. For example, research in economics has considered the incapacitative impact of school, which keeps youths out of the community and potentially reduces property crime, and of bad weather, which keeps people off the streets. It is also possible to talk about the incapacitative effect of police and even probation officers, who can detain and otherwise obstruct individuals from committing crime by their presence and actions. House arrest and electronic monitoring, which have become increasingly common in the USA, may serve as a deterrent but may also incapacitate people by making it more difficult to engage in criminal behavior. New monitoring policies which require individuals to check in daily for drug and alcohol tests may incapacitate offenders by requiring certain behavior (showing up for breathalyzer tests) when they would otherwise be drinking. Sex offender restrictions on housing locations are also another form of incapacitation. No attempt is being made to deter or change the underlying propensity of the individuals. Rather, policies are created which will make it harder for these sex offenders to actuate their underlying propensities.

These alternative forms of incapacitation might not be as complete as imprisonment, but they may also not carry with them the costs associated with concentrating large numbers of offenders in a prison. The costs of creating such potentially violent environments are not typically considered in the average incapacitation study, which only focuses on crimes in the community. In contrast, evaluations of alternative forms of incapacitation do consider the crimes that are committed while under supervision. For example, evaluations of electronic monitoring compare the behavior of people with the monitors to the behavior of people without monitors. No assumption is made that people with monitoring do not commit crime. A realistic appraisal of these new forms of incapacitation starts with a clear understanding of what the constraints placed on an individual can and cannot prevent. This same realistic appraisal could, and perhaps should, be applied to the constraint of prison. By broadening the definition of incapacitation, researchers may also start to question the fundamental assumptions underlying standard research on incapacitation.


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