B25 - The Limits of
Incapacitation as a Crime Control Strategy
1. INTRODUCTION: THE APPEAL OF
The research literature distinguishes between two types of incapacitation policy: selective incapacitation and collective incapacitation. Both involve the use of longer prison sentences on offenders, but in selective incapacitation the longer sentence applies only to those who are identified as high-rate offenders by some prediction method:
By selective incapacitation, we mean the prevention of crime through physical restraint of persons selected for confinement on the basis of a prediction that they, and not others, will engage in forbidden behaviour in the absence of confinement. By contrast, collective incapacitation refers to crime reduction accomplished through physical restraint no matter what the goal of confinement happens to be (deterrent, rehabilitative, incapacitative, etc.), and where decisions about who is to be imprisoned need not necessarily entail predictions as to future conduct (Greenberg 1975, p.542).
Selective incapacitation is considered appropriate for offenders who appear to offend with unusually high frequency. The distinction between the two types of policy is important because, as subsequent sections will show, the effect of collective incapacitation is generally estimated to be fairly limited. However, selective incapacitation holds the promise of more efficient use of expensive prison resources while achieving effective crime control.
The attractions of incapacitation go beyond the simplistic 'lock 'em up' rhetoric found in the popular media. The literature has documented its rise as a penal strategy in the early 1970s in the United States (Cohen 1983; von Hirsch 1985) following general disillusionment with the 'rehabilitation ideal'. The influential work of James Q. Wilson (1975; 1983), arguing in favour of incapacitation, found eager supporters among American citizens 'fearful and angry about rising rates of serious crime' (von Hirsch 1985, p.9). If rehabilitation efforts were ineffective and deterrence effects uncertain, incapacitation was seen as a straightforward way of at least stopping convicted offenders from committing further offences. Wilson suggested that a reduction of up to 20 per cent in the robbery rate was possible by following a strategy of imprisoning serious offenders.
This bulletin examines the research evidence on
the effectiveness of collective and selective incapacitation as crime
control strategies. Section 2 contains a brief introduction to the
technical issues underlying research in this area. The key findings of the
international literature over the past 20 years are then summarised in
Section 3. The implications of these findings are discussed in Section
2. TECHNIQUES FOR ASSESSING
THE EFFECTS OF INCAPACITATION
It has been shown that this proportion is equivalent to the percentage reduction in crime achieved by imprisonment.
If TR is large compared
with S, then
approaches unity and equation (1) reduces to:
An intuitive justification of equation (2) is provided by Cohen (1983, p.17). When TR is much larger than S, the average time between incarcerations is the reciprocal of the rate of being sentenced to imprisonment per year, 1/lqJ. If S is the average length of prison terms, the proportion of career that an offender is incapacitated is given by:
Average prison stay / (Average time between
incarcerations+ Average prison stay)
SIMULATING EFFECTS OF POLICY CHANGE
The second assumption is that offences which would have been prevented when certain offenders are in custody are not replaced by offences committed by other offenders. This assumption is more problematic, as the replacement of an offender is quite conceivable in certain situations:
This could happen if, for example, the offender were part of an organized illegal economic activity like drug sales or burglaries organized by a fence; in this event a replacement might simply be recruited from an available 'labour market' to continue the crimes that would otherwise be committed by the incarcerated offender. Alternately, if the offender were part of a crime-committing group, the remaining members of the group might continue their criminal activity, with or without recruiting a replacement (Cohen 1983, p.9).
The consequence of replacement or group offending is the reduction of the incapacitation effect of imprisonment. However, without more precise knowledge of the effects of incapacitation on the offending pattern of individual or group offenders in specific community settings, it is impossible to estimate the magnitude of the necessary adjustment (Zimring and Hawkins 1995).
The third underlying assumption is that the experience of imprisonment does not change the expected length of criminal career (T) or individual crime rate (l). In other words, the rehabilitative or criminogenic effects of imprisonment are assumed to be negligible, and the deterrent effect of imprisonment on other offenders minimal. If this assumption was violated, the incapacitative effect would not be affected in the short run, but long-run estimates are more vulnerable.
A criminogenic effect of incarceration that increases individual crime rates or lengthens careers after release would perversely lead to future increases in the incapacitative effect that could be achieved from continuing the same incarceration policies. As the mean individual crime rate or the expected career length increases, so also does the number of crimes that can be averted through incapacitation from each man-year incarcerated. Similarly, the long-run crime reduction directly associated with incapacitation would decrease if the mean individual crime rate or expected career length declined in the future as a result of rehabilitation or deterrence. Fewer crimes would be averted by the same incarceration level. In each case, the gains from one form of crime control are counteracted by losses from another form. With criminogenic effects, failure to account for changes in criminal careers would lead to long-run underestimates of the incapacitative effect; in the presence of rehabilitation or deterrence, the long-run incapacitative effect would be overestimated (Cohen 1983, pp.9-10).
Cohen has argued that while offending rates may be altered by imprisonment at the individual level, at the aggregate level criminogenic and rehabilitative effects are likely to offset each other and produce no net effect. The deterrent effect of imprisonment may lead to an overestimate of the incapacitative effect, but the importance of this distortion is diminished if the latter is small (Cohen 1983, p.10).
A related assumption to the above is the
stability of individual offending rate l over time. The notion of
'crime spurting' is a relevant consideration. This refers to irregular
offending behaviour with periods of high frequency interspersed with
periods of low frequency in offending. Evidence of spurting was found in
the Second Rand Survey (see later discussion) where 'periods of high
activity clustered just prior to the current incarceration' (Blumstein et
al. 1986, p.64; also see later discussion on Haapanen 1990). Estimates of
l based on
offending patterns immediately prior to incarceration are likely to be
exaggerated, as is the estimated incapacitative effect of
A similar estimate of elasticity can be
obtained using the simulation method. This requires, first of all,
comparing the number of years of imprisonment the sample of offenders
would serve under the hypothetical policy (S1) and the number of years actually served under the
existing policy (S0). This ratio is then divided by the percentage reduction
in crime achieved by the hypothetical policy (I). Thus:
3. THE EFFECTIVENESS OF
COLLECTIVE AND SELECTIVE INCAPACITATION
INCAPACITATIVE EFFECT OF
I. CLARKE (1974)
II. GREENBERG (1975)
III. SHINNAR AND SHINNAR (1975)
IV. PETERSON AND BRAIKER WITH POLICH
V. TARLING (1993)
VI. ZIMRING AND HAWKINS (1995)
I. GREENBERG (1975)
II. VAN DINE, CONRAD AND DINITZ (1977,
III. PETERSILIA AND GREENWOOD (1978)
IV. COHEN (1982)
V. TARLING (1993)
IMPACT ON PRISON
Cohen (1978, Appendix C) presented estimates of elasticities E for 29 States in the US. The results suggest that to achieve a ten per cent reduction in index offences, assuming an individual offending rate of l = 5 index crimes per year and an expected length of imprisonment qJS = 0.02 years, the prison population would have to be more than doubled7 (E varies considerably from 3.37 in Mississippi to 32.63 in Hawaii, with E greater than 10 in two-thirds of the States). Tarling's (1993) estimate in his study of England and Wales was even less favourable: to reduce crime by ten per cent, an increase in the use of imprisonment of 220 to 280 per cent would be required (E = 22 to 28 per cent).
To estimate the short-term impact of State
prison population8 on crime rates, Marvell and Moody (1994) used advanced
econometric time-series regression techniques on imprisonment and crime
rates for the years 1971-1989 over 49 States in the US. They estimated
that each 10 per cent increase in the State prison population resulted in
1.6 per cent fewer index crimes per year. If this figure was translated
into the elasticity defined earlier, the value of E would be 6.25. In other
words, to achieve a ten per cent reduction in index crimes, the prison
population would have to increase by 62.5 per cent. The reduction effect
estimated by Marvell and Mood varied considerably by offence type: 0.65
per cent for homicide, 1.1 per cent for rape, 2.5 per cent for burglary,
2.6 per cent for robbery, and 2.0 per cent for vehicle theft. The overall
1.6 per cent estimate turned out to be the same as the 'best estimate'
produced by Spelman's (1994) results based on the 1978 Rand Surveys which
included federal prison as well as local jail inmates (see next section
for more details on the Rand Surveys). Although Marvell and Moody
suggested that the 'real impact' of imprisonment was much greater than
their estimate of 1.6 per cent (perhaps as high as 3.3 per cent), they
cautioned that their study was limited to the short-term impact of State
prison population on index crime. Long-term impacts as well as impacts of
population changes in local jails, federal prisons and juvenile detention
facilities were not considered.
The plausibility of selective incapacitation was enhanced by studies which showed that the statistical distribution of individual offending rates l is highly skewed. In the Rand Inmate Surveys of 1976 and 1978, for example, self-reported rates of offending varied substantially: half of the prisoners reported having committed fewer than four robberies per year (while free), while about five per cent of the prisoners reported having committed more than 180 robberies per year (see Visher 1986). If these high-rate offenders could be identified and given longer prison sentences, the proportion of offences prevented would have been substantially increased with only modest increases in the prison population.
The effectiveness of selective incapacitation
has been examined by a number of studies. The results are summarised
I. THE RAND INMATE SURVEY
Three main findings from the survey are relevant to incapacitation policy. First, the survey confirmed the highly skewed distribution of the individual offending rate l. For example, the median and the 90th percentile9 values were 5.45 and 232 for burglary, 5.00 and 87 for robbery, 8.59 and 425 for theft (Visher 1986, p.167).
Secondly, the survey provided data for Chaiken and Chaiken (1982) to develop a typology of offenders using multivariate techniques.10 Six major types represented 62 per cent of the sample: violent predators (15 per cent of sample), robber ---- assaulters (8 per cent), robber -drug-dealers (9 per cent), low-level robbers (12 per cent), burglar - drug-dealers (10 per cent) and low-level burglars (8 per cent). The most active 10 per cent of violent predators reported having committed at least 154 robberies and 516 burglaries a year.
Finally, the survey provided data which allowed Greenwood (1982) to identify high-rate offenders using self-reported information. Greenwood developed a simple, seven-point scale using variables that correlated well with high rates of burglary and robbery. Respondents were given a score of one or zero depending on the presence or absence of each of the following seven attributes: convicted previously for the same charge, incarcerated more than 50 per cent of the preceding two years, convicted before age 16, served time in state juvenile facility, used drug in preceding two years, used drugs as a juvenile, and employed less than 50 per cent of preceding two years. The scores were added together and a respondent was classified as a low-rate (scoring 0 or 1), medium-rate (scoring 2 or 3), or high-rate (scoring 4 or more) offender. Greenwood used the model of incapacitation developed by Avi-Itzhak and Shinnar (1973) and suggested that a policy of selective incapacitation would have significant crime control benefits. For example, if all convicted robbers predicted by the seven-point scale to be high-rate robbers were given an eight-year prison sentence while all the other robbers were given a one-year sentence, the robbery rate could be reduced by a maximum of 20 per cent without any increase in the prison population.
Greenwood's (1982) study was enthusiastically received by policy makers and even put into practice through legislation or informal guide in some States. Several critical reviews of the study, however, raised serious ethical (von Hirsch 1985; see later section) as well as technical concerns about the results (Cohen 1983). Technical problems include the use of self-report data among convicted offenders. Any concealment or exaggeration of offending activities would have contributed to the skewness of the distribution of offending rates. The accuracy of l estimates is also questionable, since they depend on the assumption that offenders exhibit stable offending patterns over time. If even a minority of the sample of respondents operate erratically ('crime spurting'), the estimates of l may be inflated. The heavy reliance on self-reported information in Greenwood's (1982) seven-point scale was another source of criticism. If Greenwood's policy was put into practice, the prediction instrument would have to be based on incomplete official records. Cohen (1983, p.49) also found that the level of 'false positives' in Greenwood's predictive scale was as high as 55 per cent, i.e. over half of those classified as high-rate offenders were actually low- or medium-rate offenders. Greenwood's claim about the crime-reducing effect of selective incapacitation was also questioned because the prediction model was based on retrospective data without any validation on an independent sample.
Because of the policy significance of this study, the data of the Rand Survey were carefully scrutinised and re-analysed by Visher (1986). The results of Visher's re-analysis are summarised below.
The re-analysis confirmed that the distribution of l among the sample of prisoners was highly skewed, even though minor errors may have been introduced into the estimates. However, Visher found that the estimates of l for robbery and burglary were sensitive to 'choices in computation, such as the interpretation of ambiguous survey responses, the treatment of missing data, and the computation of the length of respondents' "street time"' (Visher 1986, p.204). Visher also raised doubts regarding the veracity of some respondents: the large numbers of convicted robbers (28 per cent) and burglars (30 per cent) who reported that they had not committed any robberies or burglaries in the past one to two years and the few respondents who admitted to committing the equivalent of 1,000 or more robberies and burglaries per year. With respondents who were incarcerated for long periods of time or those who had short but 'intensive' street time, it was difficult to obtain accurate 'annualised' rates of offending. Finally, Visher suggested that further research was necessary to explain the considerable variation in the values of l across the three State samples.
The accuracy of Greenwood's (1982) scale for identifying high-rate offenders was found to be disappointing even when applied to the original survey sample: 39 to 66 per cent of the high-rate offenders were incorrectly classified11 depending on the offence and the State (Visher 1986, p.195). The scale was found to provide an 'improvement over chance' (random prediction) of only 19 per cent for burglary in Michigan but 57 per cent for robbery in California. The scale was also better at identifying low-rate offenders than high-rate offenders. Visher's re-analysis also found that the incapacitation effect calculated by Greenwood (1982) was overestimated. Instead of a 20 per cent reduction in robbery, Visher found that a reduction of 13 per cent was the most that could be achieved by doubling the sentence length from 4 to 8 years for high-rate offenders. More troubling, however, is Visher's suggestion that if the same prediction scale and sentencing policy were applied to Michigan and Texas, the crime rate would in fact increase and the prison population decrease:
With 8-year sentence lengths for predicted high-rate robbers and 1-year jail terms for all other robbers, the robbery rate in Michigan would increase by 33 per cent, but the prison population would decrease by nearly 50 per cent. ... because incarcerated high-rate offenders ... are apparently a very small group in Michigan prisons and jails, compared with California. Moreover, all convicted robbers in Michigan are already serving long prison terms (an average of 5 years) and few robbers are sentenced to jail... [M]ost robbers (those defined as low- and medium-rate) would spend a smaller portion of their offending careers in prison or jail under this policy than under Michigan's current policy and would have more 'free time' in which to commit more crimes (Visher 1986, pp.201-2).
The anticipated 13 per cent incapacitative effect in California was expected to decline when applied to any new population ('shrinkage'12) and especially when applied to a population of convicted offenders rather than prisoners. The effect would also decline if official records rather than self-report data were used for prediction.
II. HAAPANEN (1990)
The study found evidence to question some of the assumptions and the results of selective incapacitation strategies. The most significant finding was that individual offending rates tend not to be stable over time. Using arrest rates as estimates of offending rates, Haapanen's analysis found that individual arrest rates showed substantial instability from one four-year period to the next. Few offenders consistently maintained the same level of arrest rate over four-year periods: 'While most of the sample had at least one four-year period in which their rates of arrest were among the highest third, only a minority of these (28% over three periods and 12% over four periods) were in the highest thirds over most of these periods' (Haapanen 1990, p.140). These results suggest that models that assume stable offending rates may overestimate the incapacitative effect of selectively locking up individuals who were identified as high-rate offenders at particular times.
Haapanen also pointed to two important patterns in his data. First, arrest rates showed a clear 'uncharacteristic' increase during the four-year period immediately prior to incarceration. This suggests that it would be inappropriate to use the arrest rates of this period to estimate the average rate of offending. Secondly, arrest rates for the years following release from imprisonment tended to be lower than expected. This trend suggests that the crime-reducing effect of extending the prison sentence might be somewhat less than predicted using pre-incarceration arrest rates.
Finally, using data on arrest and incarceration
during the 12-month period after a prisoner had been released from prison,
Haapanen estimated the potential incapacitation effect of adding one year
to the prison sentence. The results were generally less favourable than
other studies cited: keeping all
offenders in prison for an additional
year would reduced crime by only 3 per cent, while keeping those with the
highest post-prison arrest rates in prison for an additional year would
have reduced crime by less than 2 per cent.
III. BERNARD AND RITTI
Estimates of costs and benefits were based on 627 boys in the cohort who accumulated at least five police contacts. Each incapacitation policy involved holding a youth in institution from the time of an 'incapacitating event', such as the second arrest, the second adjudication or the second institutionalisation, until his 18th birthday. A police contact or felony adjudication was counted as having been prevented if it occurred following the hypothetical incapacitation.
The researchers concluded that there was no evidence that selective incapacitation was a practical strategy for controlling crime:
Our least harsh hypothetical policy would have reduced serious adjudications by 6% but only by incarcerating between two and six times as many juveniles as at present. Our most harsh policy would have reduced serious adjudications by 35% but only by incarcerating between 9 and 22 times as many juveniles as at present. In addition, the policy that 'selectively' focused on more serious offenders produced declining marginal benefits per offender,13 apparently because these offenders were already being locked up by judges (Bernard and Ritti 1991, pp.50-1).
While the research evidence from a number of studies challenged the original optimistic assessment of selective incapacitation strategies, concerns were also raised by those who found the strategy morally and philosophically objectionable. Critics argued that sentencing of offenders should be based on 'just desert' and proportionality principles: offenders should be punished according to the seriousness of the offence they were convicted for, not according to a prediction of future behaviour (see, for example, von Hirsch 1985).
The high rate of 'false positives' associated
with Greenwood's (1982) prediction model means that a substantial
proportion of low- or medium-rate offenders would be misclassified as
high-rate offenders and would receive much more severe sentences than they
would otherwise receive. The use of variables unrelated to the current
offence in the prediction scale also means that sentencing would be based
on factors not considered relevant to assessing the gravity of the
offence. One study indicated that 'disadvantaged groups in society
(blacks, women, and the poor) are more likely to receive higher Greenwood
scale scores, and thus designation as high-rate offenders, even when
controls for prior offences are included' (Decker and Salert 1987, p.287).
More recently, Long (1993) argued from a 'rights-based tradition of
political morality' that selective and collective incapacitation
strategies exceed the limits of the legitimate exercise of state powers in
a democratic society.
An evaluation of the Michigan Felony Firearm Statute14 introduced in 1977, for example, found that there was a slight increase in the average sentence for felony offences, but the proportion of all defendants receiving prison sentences did not increase. Research results suggest that 'waiver trials' were used to avoid the mandatory two-year sentence: judges either gave explicit prior indications that they would dismiss the firearm charges at trial or indicated that they would consider every possible defence and require evidence of every element of the charge. There was also evidence that judges had in fact adjusted their prior tariff to take into account the two years added by the new law.
Research on a similar law in 1975 in Massachusetts15 found evidence of a substantial increase in acquittals among defendants charged with carrying a firearm and those also charged with robbery. Appeals to the superior court showed a large increase and the rate of absconding of defendants also increased.
In another study, it was found that the introduction of severe mandatory sentences for drug offences in New York in 1973 led to a marked increase in the severity of prison sentences. However, because the law forbade dismissal of charges through plea bargaining, trial rates and court delay increased dramatically, leading to a large increase in backlog of drug cases.
These results, of course, do not imply that incapacitation policies will necessarily lead to adaptive responses by practitioners. They merely suggest that policies, even when prescribed by legislation, do not always lead to consequences intended by advocates of these policies. It is not inconceivable that judicial officers and lawyers, who play an important part in implementing such selective incapacitation policies, might respond 'in ways that reduce the disparity that arises from a sentence of 8 years for predicted high-rate offenders compared with 1 year for other convicted persons' (Visher 1986, pp.205-6).
In Western Australia, where The Crime (Serious and Repeat Offenders) Sentencing Act
1992 was introduced to target 'serious
repeat offenders' and 'repeat violent offenders', only a handful of 'hard
core' offenders16 have been incapacitated (Broadhurst and Loh 1993). One
problem identified was the uncertainty over the definition of 'conviction
appearance' specified by the Act. The controversy over the Act might also
have 'produced some tentativeness or reluctance among key players in the
criminal justice system and acted to depress enthusiasm for prosecution
under the Act' (Broadhurst and Loh 1993, p.258; see also Harding 1993). In
many ways, the symbolic power of the Act as demonstrating the 'toughness'
of the government on juvenile offenders seemed more important than its
4. CONCLUSION: THE LIMITS OF
In conclusion, it is important to point out that the models used to evaluate the effectiveness of incapacitation are based on a number of assumptions, which may in fact be false. For example, the assumption that offenders sentenced to imprisonment are not replaced by other offenders may not hold for activities such as drug trafficking or property crime where a market for the illegal substance or stolen goods exists. The assumption that individual offending rates are stable over their 'criminal careers' has also been challenged by some research findings. Any violation of these assumptions would lead to a lower estimate of the incapacitative effect of imprisonment.
Acknowledgement: I would like to
thank Don Weatherburn, Bronwyn Lind and Peter Saunders for comments on an
earlier version of this bulletin and Lise Carroll for research
Bernard, T.J. and Ritti, R. R. 1991, 'The Philadelphia Birth Cohort and Selective Incapacitation', Journal of Research in Crime and Delinquency, 28(1) pp.33-54.
Blumstein, A., Cohen, J., Roth, J. A. and Visher, C. A. (eds) 1986, Criminal Careers and 'Career Criminals', Volume I, National Academy Press, Washington, DC.
Broadhurst, R. and Loh, N. 1993, 'The Phantom of Deterrence: The Crime (Serious and Repeat Offenders) Sentencing Act', ANZ Journal of Criminology, 26(3) pp. 251-271.
Chaiken, J.M. and Chaiken, M. 1982, Varieties of Criminal Behavior, Report to the National Institute of Justice, Rand Corporation, Santa Monica, Calif. (Quoted in Visher 1986.)
Clarke, S.H. 1974, 'Getting 'Em Out of Circulation: Does Incarceration of Juvenile Offenders Reduce Crime?', Journal of Criminal Law and Criminology 65 pp.528-35. (Quoted in Cohen 1983.)
Cohen, J. 1983, 'Incapacitation as a Strategy for Crime Control: Possibilities and Pitfalls' in M. Tonry and N. Morris (eds) Crime and Justice: An Annual Review of Research, Volume 5, Chicago, University of Chicago Press, 1983 pp.1-84.
Cohen, J. 1982, Patterns of Adult Offending, Ph.D. dissertation, Carnegie-Mellon University. (Quoted in Cohen 1983.)
Cohen, J. 1978, 'The Incapacitative Effect of Imprisonment: A Critical Review of the Literature', in Blumstein et al. (eds) 1978, Deterrence and Incapacitation, National Academy of Sciences, Washington, DC, pp.187-243.
Cohen, J. and Tonry, M. H. 1983, 'Sentencing Reforms and Their Impacts' in Blumstein, A., Cohen, J., Martin, S. and Tonry, M. (eds) 1983, Research on Sentencing: The Search for Reform, Volume II, National Academy Press, Washington, DC, pp.305-459.
Decker, S.H. and Salert, B. 1987, 'Selective Incapacitation: A Note on Its Impact on Minorities', Journal of Criminal Justice, 15 pp.287-299.
Greenberg, D.F. 1975, 'The Incapacitative Effect of Imprisonment: Some Estimates', Law and Society Review, 9 pp.541-80.
Greenwood, P.W. 1982, Selective Incapacitation, Report to the National Institute of Justice, Rand Corporation, Santa Monica, Calif.
Haapanen, R. 1990, Selective Incapacitation and the Serious Offender: A Longitudinal Study of Criminal Career Patterns, Springer-Verlag, New York:
Harding, R. (ed) 1993, Repeat Juvenile Offenders: The Failure of Selective Incapacitation in Western Australia, Crime Research Centre, University of Western Australia, Perth.
Long, L.J. 1993, 'Rethinking Selective Incapacitation: More At Stake Than Controlling Violent Crime', University of Missouri at Kansas City Law Review, 62 pp.107-70.
Marvell, T.B. and Moody, C.E. Jr 1994, 'Prison Population Growth and Crime Reduction', Journal of Quantitative Criminology, 10(2) pp.109-140.
New South Wales Department of Corrective Services 1994, 1993-94 Annual Report, New South Wales Department of Corrective Services, Sydney.
Petersilia, J. and Greenwood, P. 1978, 'Mandatory Prison Sentences: Their Projected Effects on Crime and Prison Populations', Journal of Criminal Law and Criminology, 69 pp.604-15.
Peterson, M.A. and Braiker, H. B., with Polich, S. M. 1980, Doing Crime: A Survey of California Prison Inmates, Report R-2200-DOJ, Rand Corporation, Santa Monica, Calif. (Quoted in Cohen 1983.)
Shinnar, R. and Shinnar, S. 1975, 'The Effect of the Criminal Justice System on the Control of Crime: A Quantitative Approach', Law and Society Review 9 pp.581-611.
Spelman, W. 1994, Criminal Incapacitation, Plenum Press, New York. (Quoted in Marvell and Moody 1994.)
Tarling, R. 1993, Analysing Offending: Data, Model and Interpretation, HMSO, London.
Van Dine, S., Dinitz, S. and Conrad, J. P. 1977, 'The Incapacitation of the Dangerous Offender: A Statistical Experiment', Journal of Research in Crime and Delinquency 14 pp.22-35. (Quoted in Cohen 1983.)
Van Dine, S., Conrad, J. P. and Dinitz, S. 1979, Restraining the Wicked: The Dangerous Offender Project, Lexington Books, Lexington, Mass.
Visher, C.A. 1986, 'The Rand Inmate Survey: A Reanalysis', in Blumstein et al. (eds) 1986, Criminal Careers and 'Career Criminals', Volume II, National Academy Press, Washington, DC, pp.161-211.
von Hirsch, A. 1985, Past or Future Crimes: Deservedness and Dangerousness in the Sentencing of Criminals, Rutgers University Press, New Brunswick, NJ.
Wilson, J.Q. 1975, Thinking About Crime, Basic Books, New York. (Second Edition 1983)
Wolfgang, M.E., Figlio, R. M. and Sellin, T. 1972,
Delinquency in a Birth Cohort, University of Chicago Press, Chicago.
Zimring F. and Hawkins, G. 1995, Incapacitation: Penal Confinement and the Restraint of Crime, Oxford Univerity Press, New York.