This study investigated victimisation and offending histories in a sample of people with intellectual disability and a community comparison sample using a case linkage design. The results indicated that, overall, people with intellectual disability were less likely to have an official history of victimisation and were no more or less likely to have a history of criminal offending than people without intellectual disability. Of note, however, the ID group were significantly more vulnerable to violent and sexual victimisation and offending compared to the community.
These findings suggest that members of the general community are more likely to have a police record as a victim of crime overall, with the vast majority of these crimes being non-violent and non-sexual in nature. This finding is consistent with some prior research [2, 38], but contrary to previous theory and other research, which has suggested pronounced vulnerabilities for people with ID across all crime types [5, 6, 10, 18]. Two explanations may account for this finding. First, individuals in the ID sample may have had less exposure to certain types of victimisation experiences due to the nature of their community and/or residential living circumstances. Alternatively, non-violent non sexual crimes may be under-reported by people with ID, who may not be aware of appropriate avenues for reporting, may be unable to recognise more ambiguous non-violent non-sexual crimes, may not be progressed to police services by carers/residential staff, or that they may fear reporting a person who they depend on .
Baladerian and others  noted that less than half of violent and sexual crimes against people with ID were reported to police, and of those reported, over half said nothing happened and less than 1 in 10 perpetrators were subsequently arrested. Participants in that study cited a lack of confidence in the criminal justice system, fear of retribution and poor knowledge of reporting avenues as key barriers to reporting. The potential for underreporting should be considered. Practically, efforts to support vulnerable populations report crime should be considered.
Despite the community having a higher rate of victimisation for crime overall, the current findings demonstrate that serious offences such as violent and sexual crimes are statistically more prevalent among people with ID, which supports the study hypotheses and previous research findings [11, 13, 19–22]. Roughly 1 in 6 people in the ID sample had reported violent victimisation to police, which was twice the rate of the community; furthermore 6 % had reported sexual victimisation, which was nearly six times higher than the community rate. These figures are concerning and provide robust epidemiological insight into the extent of an under researched problem and supports the conclusions of previous smaller scale studies which noted the heightened risk for violent and sexual crimes specifically [6, 13, 19].
Of particular interest to the current study was the influence of gender on victimisation. Findings indicate that males in the community group were more likely to have records of being victims than females, consistent with prior research [25, 39–41]. Interestingly, this pattern of victimisation was not evident in the ID sample where there was no difference among victimisation types for gender, except for violent victimisation, which was significantly higher for females compared to males. This result is substantiated by the large difference in violent and sexual victimisation between the ID group and the community, for females more so than males, suggesting that females with ID are an especially vulnerable subgroup. What makes them specifically vulnerable to violent crime is unclear in the literature with only one report highlighting a possible gender difference . The findings from the present study, coupled with the continued lack of consensus in the literature, should act as a catalyst to focus more on elucidating potential differences, and if so why these exist, as at present females with ID are particularly vulnerable to serious victimisation.
The rate of criminal offending, in this sample operationalized as criminal charges being laid, was entirely consistent with previous studies [14, 33]. In this study, the overall rate of offending did not differ between the intellectual disabled and the community groups, with less than 10 % of both samples having an official record of offending. However, like victimisation, violent and sexual offending were statistically more common for people with ID, with offending six and a half times higher for violent crime and nearly 19 times higher for sexual crimes. This result can be interpreted using Routine Activities Theory  which postulates that a greater exposure to crime-inducing situations and personal reactions from the person with ID can make them more vulnerable to victimisation. This can also be applied to offending situations where an individual is similarly exposed to dangerous situations and there is still a potential for the individual to be a victim or an offender in an ambiguous and threatening situation. Empirical research corroborates this theoretical assumption and the current findings, which note a disproportionate number of people with ID in the criminal justice system and suggest their particular susceptibility to sexual offending [2, 14, 33, 34]. In line with the Routine Activities Theory  the higher rates of offending may be related to the significant environmental and individual challenges faced by people with ID . While offending may be more pronounced in people with ID, there is also a greater propensity for parental adversity, low socio economic status and mental illness. Future research should therefore seek to discern the relative importance of these variables to crime in ID populations.
Comorbid mental illness
The presence of a co-occurring mental illness significantly increased the likelihood of people with ID having both victimisation and offending histories. Mental illness has been associated with victimisation and offending in other vulnerable populations [42–45] with one study suggesting that mental illness may be linked with criminality in people with intellectual disability . The current findings suggest that mental illness complicates the association between ID and victimisation and offending considerably (almost doubling the rates of both).
The presence of a mental illness may further limit the functionality of the individual with ID and may intensify their exposure to dangerous situations and reactions to potential perpetrators. The association between ID with comorbid mental illness and increased rates of victimisation and offending indicates that treating mental illness alongside managing deficits associated with ID could have beneficial effects for crime prevention and victimisation. However, while mental illness appears to be pivotal in influencing susceptibility to crime, there may be other contributing factors such as substance abuse, outside of the scope of the present study, which further complicates the relationship.
The present study complements the existing literature base providing a robust contemporary prevalence estimate for victimisation and offending among people with ID. It adds further weight to prior research findings regarding criminality among those with ID and proposes that people with ID are at a significant disadvantage and are over represented in crime figures. By illustrating the magnitude of the problem, using a robust epidemiological design, it is hoped that there will be greater research into why this problem exists and how this effect can be minimised.
The findings in the current study may be limited by several factors that were inherent in the databases used. First, the Restrictive Interventions Database System (RIDS) may not be representative of all people with intellectual disabilities as individuals who are included on this reporting system have been subject to at least one restrictive intervention. This may have led to an over or underestimation of the true rates offending and victimisation reported here. While not available through the current dataset, taking into account a frequency criterion relating to the number of restrictive intervention episodes experienced by the person with ID as a potential confounding factor may help further develop our understanding of both risks and vulnerabilities to crime among people with ID who have more complex presentations or who present with more challenging behavioural management issues. Second, notwithstanding the challenges associated with diagnosing mental disorders among people with ID [47, 48], rates of mental illness were estimated from a public mental health database which, as noted in the methodology, under-reports some of the more high prevalence disorders. That being said, a strength with the current methodology was that case ascertainment of mental disorder and police contact for individuals in the community comparison sample was identical, thereby leading to direct comparability and greater confidence in the magnitude and direction of associations reported here. Thirdly, the findings may underrepresent the extent of crime involvement in ID as victimisation and offending data were based on contact with the police, where official reports were made and subsequently recorded. From the literature, we know that individuals with ID typically under report crimes and may find it difficult to recognise these crimes. It is likely that the current estimate of crime is more conservative compared to true prevalence figures . Further, it was not possible to statistically control for the potentially confounding effect of age as the ID and community databases were independent of each other. While this is unlikely to affect the direction of the associations with violent and sexual offending, the younger age of the ID sample may explain the finding pertaining to their lower risk of being the victim of other types of non-sexual non-violent crimes found with this sample, although the evidence remains inconclusive. Lastly, the community comparison sample operationalized offending at the level of conviction, while the ID sample used the level of criminal charge; this may mean that the statistical differences presented may represent an upper confidence limit for estimates increased risks of both perpetration and victimisation histories.
Directions for future research
Future research should seek to replicate and extend on current findings, which represent a preliminary yet robust insight into the vulnerabilities of those with ID. Of particular interest is future research is differences between specific age groups, mental illnesses and specific licit and illicit substances. Mental illness was a key factor in the association between intellectual disability and crime perpetration, the influence of specific mental illness on ID was not considered in the current study, as diagnoses are difficult to establish with comorbid ID. A future study with a greater focus on the robust assessment of mental illnesses and substance use could identify more specific disorders that are pertinent to both victimisation and offending; such information would be critical to informing both risk assessment and treatment planning.