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Introduction
It is not clear whether people with intellectual disabilities (ID) commit more crime than those without ID, or, in fact, whether the nature and frequency of offending by people with ID differs from that committed by offenders in the general population. The ambiguity concerning these issues is due in large part to methodological problems in prevalence studies in this area (Lindsay and Taylor, 2010). One source of variation in prevalence of offending reported across studies is the location of the study (community, prison–remand, prison–sentenced, hospital–high/medium/low-secure), which can result in sampling bias and filtering effects. Frequently, the inclusion criteria used in prevalence studies vary or are not clear and this can affect the rates obtained – particularly if people with borderline intelligence are included. Also, the method used to identify ID (standardized vs. screening IQ tests, educational history, clinical assessment) can have a significant impact.
These methodological issues are well illustrated in the research literature concerning the prevalence of offenders with ID in prisons. MacEachron (1979) reviewed the literature for prevalence of offenders with ID in prisons in the USA and found a range of 2.6–39.6%. Fazel et al. (2008) reviewed 10 well-conducted studies of prevalence of ID in prisons (remand and sentenced) conducted between 1988 and 1997 in five common law countries and reported rates between 0% and 2.9%.
More recently, Crocker et al. (2007) assessed 281 pretrial prisoners in Montreal, Canada using three subscales of a standardized ability scale and reported that 18.9% were in the ID range. Søndenaa et al. (2008) used an IQ screening assessment with 143 prisoners in Norway and found that 10.8% fell in the ID range. Contrast these findings with a study of prisoners in Victoria, Australia in which Holland and Persson (2011) found a prevalence of less than 1.3% using the Wechsler Adult Intelligence Scale; and the results of a study of 389 remand prisoners in Scotland that indicated that less than 0.5% of those assessed had ID (Davidson et al., 1995). It is difficult to reconcile these findings without concluding that the location, assessment, and sampling methods account for the variance in large part.
Recidivism
Studies of recidivism rates for offenders with ID are also affected by similar methodological problems. Reported rates tend to be high but vary considerably depending on research setting, procedures, and definitions of reoffending used (Linhorst et al., 2003). For example, Lund (1990) found a reoffending rate of 67–72% in a follow-up study involving 155 Danish offenders with ID who had been detained on statutory orders – while Klimecki et al. (1994) reported a reoffending rate of 41.3% for 75 released prisoners with ID in Victoria, Australia. More recently, Linhorst et al. (2003) reported that 25% of 252 offenders with developmental disabilities who completed a case management community program were rearrested within a six-month period following case closure; 43% of those who dropped out of the program were rearrested during the same period.
Due to the lack of controlled studies involving ID and non-intellectual disabilities offenders, it is difficult to make direct comparisons of recidivism rates. It would appear, however, that recidivism rates for offenders with ID are no higher than those for populations of general offenders. Gray et al. (2007) conducted a two-year follow-up of 145 offenders with ID and 996 offenders without ID, all discharged from independent sector hospitals in the UK. The ID group had a lower rate of reconviction for violent offenses after two years (4.8%) than the non-ID group (11.2%). This trend held true for general offenses also (9.7% and 18.7% for the respective groups).
Intellectual functioning and offending
Historically, low intellectual functioning was viewed as a determinant of criminal behavior. Scheerenberger (1983) described in detail the historical association between low intelligence and crime in the late 19th and early 20th centuries. Famously, Terman (1911) stated that “There is no investigator who denies the fearful role of mental deficiency in the production of vice, crime and delinquency ….” Goddard (1921) suggested that up to 50% of people in prisons were “mentally defective,” and later still Sutherland (1937) concluded that 50% of delinquents residing in prisons were “feeble-minded.” People with lower levels of intellectual functioning were often considered a threat to society up to the middle of the last century.
In fact, the research evidence supporting the relationship between IQ and offending is robust (West and Farrington, 1973; Hirschi and Hindelang, 1977; Goodman et al., 1995), with those in lower IQ groups having greater rates of offending than those in higher functioning groups. Even when socioeconomic status is controlled for, offending behavior has been found to be significantly related to IQ (e.g., Moffit et al., 1991; Farrington, 1995). However, most studies involve participants with IQs in the 80–120 range and there is some evidence that when participants with IQs around 1–2 standard deviations below the mean (<80) are included, the relationship with offending is less straightforward. For example, McCord and McCord (1959) found that, while the offending rate for those in the low-average IQ group (81–90) was higher than that for those with above average IQ, those in the lowest IQ group (<80) had an offending rate lower than that for the low-average group. More recently, Mears and Cochran (2013) reported data from the National Longitudinal Survey of Youth project in the USA that indicated that the relationship between IQ and offending is curvilinear, with lower IQs (<85) associated with lower levels of offending. Thus, while there would seem to be a clear relationship between offending rates and intellectual functioning, when studies are extended to include people with IQs below 80–85, the relationship does not appear to be simple or linear.
Nature of offending behavior
A policy of deinstitutionalization has been implemented across the Western world since the mid 1970s and has had a significant effect on services for people with ID who offend or engage in offending-type behavior. Such people are now more likely to be living in community settings where their offending and offending-type behavior is more visible and subject to scrutiny by the criminal justice system. This phenomenon is demonstrated by Lund (1990) in a study of 123 offenders with ID on statutory care orders in Denmark, which found a 2–3 times increase in the incidence of sex, violence, and arson offenses when comparing sentencing in 1973 and 1983. Lund suggested that these increases were less to do with an increase in offending per se in this population, but a result of deinstitutionalization policies during this period, whereby people with ID were no longer detained in hospital for indeterminate lengths of time, but living in the community where their offending behavior was more likely to be subject to normal legal processes.
In the Northgate, Cambridge, and Abertay Pathways (NCAP) project, O’Brien et al. (2010) reported on the offense characteristics of 477 adults with ID referred to ID services in three regions of the UK during a 12-month period because of antisocial or offending behavior. They found that aggression (physical and verbal) accounted for over 80% of the antisocial and offending behavior referred, with sex offenses (contact and non-contact) making up almost 30%. Just 4% of referrals concerned firesetting, whilst 19% were related to property damage. Despite the majority of those referred having significant histories of offending behavior, just under a third were found to have any kind of structured care plan.
Since most ID institutions in the UK have now closed, or reduced in capacity significantly, people with ID who offend and require secure provision are often sent to out-of-area facilities, with a consequent drain on the resources available to local ID services (Crossland et al., 2005). This is an important issue, as Carson et al. (2010) found in the NCAP study that 50% of referrals for offending behavior were to generic ID community services, and these were received mainly from primary and secondary healthcare and social services. Compared to referrals to specialist forensic ID services (including secure services), referrals to community ID services included significantly more women, were slightly older, and included significantly more people with IQ levels below 50. In contrast, those referred to specialist forensic ID services were more often referred by the courts, offender management, and tertiary healthcare services, had committed contact sex offenses, had been charged in connection with their index offenses, and had significantly higher levels of mental illness (psychoses, personality disorder, attention-deficit/hyperactivity disorder, and conduct disorder).
The early findings from service referrals and pathways research suggest that ID offenders require a range of community and specialist inpatient services available to meet their needs, and to manage the risks they present to others and to themselves. In particular, resources need to be invested in helping community services staff to develop their knowledge and skills in managing these clients using structured care plans informed by risk needs assessments.
Interventions for offenders with ID
Risk assessment
Whilst not a therapeutic intervention in the traditional sense, risk assessment is the cornerstone of therapeutic endeavor aimed at reducing offending behavior and is the foundation for clinical formulations of treatment need. Significant advances in the development of measures designed to accurately predict future violence and sexual aggression have now been extended to include offenders with ID. For example, Quinsey et al. (2004) demonstrated that the Violence Risk Appraisal Guide (VRAG), one of the best-established actuarial risk measures in the general offender literature, has good predictive accuracy when used with ID offenders. Gray et al. (2007) conducted a more extensive investigation into the VRAG. They compared 145 patients with ID and 996 mainstream patients all discharged from hospital having been admitted due to serious offending-type behavior. They found that the VRAG predicted reconviction rates in the ID sample with an effect size as large as that for the non-ID sample.
This important research on the assessment and management of risk in offenders with ID has continued in a study (the “212 Multi-Centre Risk Study”) involving 212 clients across a range of security settings: hospital high, medium and low-security, and community forensic ID services (Hogue et al., 2006). The most complex presentations, in particular comorbid personality disorder, were found in the more secure samples. Lindsay et al. (2008) combined the total cohort of offenders with ID from the 212 risk study to evaluate the predictive validity of a range of static and dynamic risk assessments. They found that the VRAG, the Historical, Clinical, Risk Management-20 (HCR-20) (Webster et al., 1997), the Short Dynamic Risk Scale (SDRS; Quinsey, 2004), and the Emotional Problems Scale (Prout and Strohmer, 1991) all showed significant “areas-under-the-curve” (AUC) (using receiver-operator characteristics (ROC) analyses) in relation to the prediction of violent incidents. The Static-99 sex offender risk assessment instrument (Hanson and Thornton, 1999) also showed a significant area-under-the-curve in relation to the prediction of sexual incidents.
Dynamic risk assessments (e.g., the SDRS) contrast with static risk assessments (e.g., the VRAG) in that the variables are amenable to change through treatment and management of the individual. As there seemed to be strong relationships between dynamic risk factors and future incidents for this client group, Lofthouse et al. (2014) reanalyzed the risk assessment data published by Lindsay et al. (2008) that showed that both the VRAG (an actuarial assessment) and the SDRS (an easy and-quick-to-complete dynamic risk assessment) had equivalent risk predictive values of AUC = 0.71 and 0.72, respectively. They investigated the functional relationships between VRAG and SDRS items to determine whether they were independent, mediating/moderating, or acting as a proxy and found that the dynamic variables on the SDRS acted as a proxy for the VRAG variables. They concluded that since these risk factors captured elements of the same underlying risk construct associated with violence, and as dynamic variables are more accessible and clinically meaningful, dynamic assessment, in the form of the SDRS, could provide more immediate and clinically relevant information to manage patients’ risk needs.
An alternative approach to understanding forensic risk has been advanced by Wheeler et al. (2014) in a study examining the impact of social and environmental variables on offending behavior by people with ID. Lack of work or routine activity, serious problems with family relationships, and exposure to antisocial or abusive friends were found to be strongest predictors of offending behavior in people with ID referred to community ID teams. This approach echoes Willis and Grace’s (2009) work on non-ID sexual offenders that showed that discharge planning, including organization (or lack of it) of accommodation, employment, and social support, predicted recidivism equally as well as established criminogenic variables.
This research has demonstrated that there are several well-established actuarial, dynamic, and clinical instruments, some of which have been developed specifically for this client group, that have good reliability, discriminative validity, and predictive validity with offenders with ID. If we can confirm that proximal dynamic indicators of risk not only have more clinical utility but are also as predictive as established static risk indicators, this could have a significant impact on developing practice to help offenders with ID to access better services in the least restrictive environments.
Anger and aggression
Research on several continents has found high rates of aggression amongst people with ID – with much higher rates for those living in institutional and secure forensic facilities than for those residing in community settings (Taylor and Novaco, 2013).
While it is neither necessary nor sufficient for aggression to occur, anger has been shown to be strongly associated with and predictive of violence in men with ID and offending histories (Novaco and Taylor, 2004). Thus, anger has become a legitimate therapeutic target. The treatment of anger and aggression using cognitive-behavioral therapy (CBT) has been extensively evaluated with a range of clinical populations (see Taylor and Novaco, 2005 for a review). One potential advantage of CBT anger treatment over interventions based on applied behavior analysis, is that self-actualization through the promotion of portable and internalized control of behavior is intrinsic to the skills-training components of these approaches (Taylor et al., 2002b). Further, there is evidence from studies in non-disability fields that for a range of psychological problems the effects of CBT are maintained and increase over time compared to control conditions (Taylor and Novaco, 2005).
Willner (2007) reviewed nine controlled studies involving people with ID that compared CBT for anger control problems with wait-list control conditions. Most of these interventions were based on the treatment approach developed by Novaco (1975) that incorporates Meichenbaum’s (1985) stress inoculation paradigm. All of these studies reported significant improvements on outcome measures for those in treatment conditions that were maintained at 3–12-month follow-up. Nicoll et al. (2013) systematically reviewed 12 studies of CBT for anger in adults with ID published between 1999 and 2011. Nine studies were included in a meta-analysis that yielded a large uncontrolled effect size (ES) (average ES = 0.84).
Taylor and colleagues have evaluated individual CBT anger treatment with detained male patients with mild–borderline ID and significant histories of violence in a linked series of studies (Taylor et al., 2002a, 2004a, 2005). The 18-session treatment package included a 6-session broadly psychoeducational and motivational preparatory phase; followed by a 12-session treatment phase based on individual formulation of each participant’s anger problems and needs, following the classical CBT stages of cognitive preparation, skills acquisition, skills rehearsal, and then practice in vivo. These studies showed significant improvements on self-reported measures of anger disposition, reactivity, and imaginal provocation following intervention in the treatment groups compared with scores for the control groups, and these differences were maintained for up to four months following treatment.
The impact of these anger interventions on aggressive behavior, including physical violence, has been investigated empirically on only a few occasions. Allan et al. (2001) and Lindsay et al. (2003) reported reductions in violence following a group intervention in a case series of six women and six men with violent convictions living in the community. In a larger study involving 47 people with ID and histories of aggression, Lindsay et al. (2004a) showed that, following a community-group anger intervention, 14% of participants had been aggressive during follow-up, compared with 45% of people in a control condition.
Novaco and Taylor (2015) described an evaluation of the impact of the cognitive-behavioral anger treatment described earlier (e.g., Taylor et al., 2005) on violent behavior by offenders with ID living in secure forensic hospital settings. The participants in this study were 44 men and six women referred by their clinical teams for anger treatment on the basis of their histories of aggression and/or current presentation. The total number of physical attacks against staff and patients fell from 319 in the12-months before treatment to 153 in the 12-month period following treatment. This represents a reduction after treatment of 52%. Importantly, the reduction in physical assaults was associated with measured reductions in anger over the course of treatment as indexed by several anger measures validated for use with this population.
In summary, there is an emerging research evidence base that CBT anger interventions can be effective in the treatment of offenders with ID and histories of aggression and violence in terms of improvements on self-report and informant anger-dependent measures that are associated with significant reductions in the number of violent incidents recorded following treatment.
Sexually aggressive behavior
Psychological treatment interventions for sex offenders with ID were reviewed by Courtney and Rose (2004). Nineteen studies published post 1990 were reviewed. These included drug treatment, problem solving, psychoeducational, and cognitive-behavioral approaches. Eleven studies were single case or small case series designs – four involved drug therapy and seven involved psychological interventions. The reported outcomes in these studies were generally positive. Eight “larger” studies, including one drug therapy, three service/management interventions, and five psychological treatments, were also reviewed. In terms of outcomes, psychological interventions appeared to be marginally superior. Eight of the studies involved group therapy interventions which were found to yield mixed outcomes, with reported recidivism rates ranging between 0% and 40%. Based on this review, and that by Lindsay (2002), it appears that most treatment approaches show some promise, but the studies were quite limited – involving small, heterogeneous samples, utilizing measures with limited reliability and validity, using poorly defined outcomes, and incorporating treatment interventions that were often described poorly. The main methodological shortcoming, however, is the absence of any controlled studies. This is due primarily to the ethical difficulty in withholding a potentially beneficial treatment given the social and legal issues involved.
More recently there has been some support for the use of cognitive and problem solving techniques in therapy for sex offenders with ID that have been shown to be effective in reducing reoffending rates in the mainstream sex offender field (see Hanson et al., 2002). Support for the centrality of cognitive distortions in the sex offending perpetrated by people with ID came from a qualitative study of nine male sex offenders by Courtney et al. (2006). They concluded that all aspects of the offense process were linked to offender attitudes and beliefs, such as denial of the offense, blaming others, and seeing themselves as the victim. Lindsay et al. (1998a–c) reported on a series of case studies with offenders with ID using CBT in which various forms of denial and mitigation of the offense were challenged over treatment periods of up to three years. Strategies for relapse prevention and the promotion of self-regulation were also component parts of the treatment. Across these studies, participants consistently reported positive changes in cognitions during treatment as assessed using the Questionnaire on Attitudes Consistent with Sexual Offenses (QACSO; Lindsay et al. 2007). More importantly, lengthy follow-up of these cases over 4–7 years showed that none had reoffended following initial conviction.
Rose et al. (2012) reported on a six-month treatment group for 12 sex offenders with ID who were living in community settings. Efforts were made to involve aspects of the offender’s broader social life into treatment by inviting carers to accompany participants. They found significant improvements on the QACSO, locus of control, and sexual knowledge and attitudes. One participant committed a sexual offense during an 18-month follow-up period.
Murphy et al. (2010) conducted a multisite study of group sex offender therapy with 46 men living in community and secure hospital settings. Following the 12-month weekly treatment schedule, significant improvements were found on the QACSO and other measures, sexual attitudes, knowledge, and victim empathy. Improvements on the QACSO and sexual knowledge were maintained at six-month follow-up. Three patients (6.5%) reoffended during the treatment program and four patients (8.7%) reoffended by carrying out 11 offenses during the six-month follow-up period. Thirty-four participants from this study were followed up for an extended period of between 15 and 106 months (Heaton and Murphy, 2013). Significant improvements on the QACSO, sexual knowledge, and victim empathy were maintained at follow-up in this study; however, eight participants (24%) reoffended during the follow-up period.
There have been a number of sex-offender treatment studies involving comparison groups with ID, although they all fall well short of adequate experimental standards, and it is important to consider the study results in light of their methodological shortcomings. Lindsay and Smith (1998) compared seven patients who had been in treatment for two or more years with another group of seven patients who had been in treatment for less than one year. Participants in the group that had been in treatment for less than one year showed significantly poorer progress and were more likely to reoffend than those treated for at least two years. Despite the small numbers involved, this study suggested that shorter treatment periods might be of limited value for this client group. This was supported by the results of a study by Rose et al. (2002) involving five men with ID living in community settings who underwent a relatively short 16-week group sex offender treatment program. No significant pre–post intervention differences were found on the QACSO or victim empathy measures.
In a further series of comparison studies involving larger groups, Lindsay and colleagues have compared individuals with ID who have committed sexual offenses with those who have committed other types of offenses. Lindsay et al. (2004b) compared 106 men who had committed sexual offenses or sexually abusive incidents with 78 men who had committed other types of offenses or serious incidents. There was a significantly higher reoffending rate in the non-sex offender cohort (51%) when compared to the sex offender cohort (19%). In a subsequent, more comprehensive evaluation, Lindsay et al. (2006) compared 121 sex offenders with 105 other types of male offenders. Reoffending rates were reported for up to 12 years after the index offense. The difference in reoffending rates between the groups was highly significant, with rates of 23.9% for male sex offenders and 59% for other types of offenders. The same authors also investigated harm reduction by examining the number of offenses committed by recidivists. They found that for those participants who reoffended, the number of offenses at up to 12 years follow-up was a quarter to a third of those recorded before treatment. This indicated a considerable harm-reduction effect following the intervention.
In a follow-up of community forensic service clients, Lindsay et al. (2013b) followed up 156 sex offenders and 126 non-sex offenders for up to 20 years. All but 15 of the study participants continued to have some access to the wider community throughout the follow-up period. Sixteen percent of sex offender participants reoffended during this lengthy follow-up period. Analyses showed that approximately 50% of the reoffending in the sex offender group occurred within the first 12 months following treatment. Some participants continued to reoffend for up to nine years post treatment; however, after that point there was no further reoffending recorded in the sex offender group up to 20 years post treatment. Comparing the number of sex offenses committed prior to referral with the offenses recorded during the 20-year follow-up period, a 95% harm-reduction effect was calculated.
Based on the limited evidence available, it is possible to conclude, albeit tentatively, that psychologically informed and well-structured interventions appear to yield reasonable outcomes in the treatment of sex offenders with ID. CBT appears to have a positive effect on offense-related attitudes and cognitions, sexual knowledge, and victim empathy. Longer periods of treatment may result in better outcomes that are maintained for longer periods. Comparison studies, although limited methodologically, indicate that psychological interventions may significantly reduce recidivism rates in sex offenders with ID – and where recidivism does occur, treatment may result in significant harm-reduction effects.
Firesetting behavior
It has been suggested that arson is over-represented in offenders with ID (e.g., Raesaenen et al., 1994). However, there is no clear research evidence to suggest that firesetting behavior is more prevalent amongst people with ID than in the general population, or whether people with ID are over-represented in the arson offender population.
Despite the alarm that firesetting behavior causes in society, there are limited published studies concerning therapeutic interventions for adults who set fires in general, and the research literature involving firesetters with ID is even sparser. Rice and Chaplin (1979) delivered a social-skills training intervention to two groups of five firesetters in a high-security psychiatric facility in North America. One of the groups was reported to be functioning in the mild–borderline ID range. Following treatment, both groups were reported to have improved, and following discharge none of the treated patients had been reconvicted or suspected of setting fires at 12 months follow-up. Clare et al. (1992) reported a case study involving a British man with mild ID who had prior convictions for arson and making nuisance telephone calls to the emergency services. Significant clinical improvements were reported following a multifaceted self-control and skills-training intervention. The client was discharged to a community placing and did not engage in any fire-related offending behavior at 30 months follow-up.
Hall et al. (2005) described the delivery of 16-session group CBT to six male patients with ID and histories of firesetting detained in a hospital medium-secure unit in the UK. Unfortunately, outcome data were not provided, although most group participants were reported to have responded positively to the intervention in terms of their clinical presentations, and two patients were successfully transferred to less secure placements following completion of the program.
Taylor et al. (2004b) reported on a case series of four detained men with ID and convictions for arson offenses. They received a broadly CBT, 40-session, group-based intervention that involved work on offense cycles, education about the costs associated with setting fires, training of skills to enhance future coping with emotional problems associated with previous firesetting behavior, and work on personalized plans to prevent relapse. The treatment successfully engaged these patients, all of whom completed the program delivered over a period of four months. Despite their intellectual and cognitive limitations, all participants showed high levels of motivation and commitment, which was reflected in generally improved attitudes with regard to personal responsibility, victim issues, and awareness of risk factors associated with their firesetting behavior. In a further series of case studies on six women with mild–borderline ID and histories of firesetting, Taylor et al. (2006) also employed the same group intervention to successfully engage participants in the therapy process. All participants completed the program and scores on measures related to fire-treatment targets generally improved following the intervention. All but one of the treatment group participants had been discharged to community placements at two-year follow-up and there had been no reports of participants setting any fires or engaging in fire risk-related behavior.
Using the same assessment and treatment approach as that used by Taylor and colleagues above, Taylor et al. (2002b) investigated the outcomes for 14 men and women with ID and arson convictions. Study participants were assessed pre and post treatment on a number of fire-specific, anger, self-esteem, and depression measures. Following treatment, significant improvements were found in all areas assessed, except for depression. Taylor (2014) reported on a follow-up of 24 firesetters with ID who had completed a specialist group treatment program. The follow-up period ranged between 4 and 13 years post treatment, at which point there had been no further arrests or convictions for arson in this cohort. File data available for 17 study participants showed that prior to treatment that subgroup had been responsible for setting a total of 425 fires. This suggests that the group-firesetters intervention used by Taylor et al. above is associated with a significant harm-reduction effect.
The results of these small and methodologically weak pilot studies do provide some limited encouragement and guidance to practitioners concerning the utility of group-based interventions for firesetting behavior by people with ID. These CBT-orientated approaches are associated with significant improvements on firesetter-specific and clinically relevant measures and reductions in firesetter behavior following treatment.
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