People with an intellectual disability (ID) are a marginalised and vulnerable group. The available research suggests an association between ID and criminal offending [
1‐
3]; this has served to propel public fear and reinforce perceptions of the need for social distance. However, the evidence from which these conclusions have been drawn remains far from definitive, with significant methodological limitations marring what are arguably tentative conclusions [
1,
2,
4]. A related area that has received much less scientific attention is criminal victimisation, despite a compelling argument that specific deficits in interpersonal functioning and cognitive capability potentially increase exposure to dangerous situations, therefore contributing to the likelihood of criminal victimisation [
5‐
14].
Intellectual disability and criminal victimisation
Intellectual Disability is characterised by significant impairments in intellectual functioning alongside difficulties in daily tasks, personal responsibility and communication [
15,
16]. From a theoretical standpoint, Routine Activities Theory [
17] conceptualises victimisation in relation to an interaction between an available victim, the absence of a capable guardian, and a motivated offender. It reasons that people with similar lifestyles or routine activities face similar victimisation risks as they are exposed to risky places and potential offenders [
5]. Some research has suggested that people with ID are most commonly victimised by their carers [
5]. Routine Activities Theory would argue that victims are easily accessible in their home / living environment (availability) and there is less protection of the victim if the perpetrator
is the guardian (absence of a capable guardian); the carer offender may be motivated to offend due to carer stress, a provocative or frustrating incident, in this context offending may be facilitated by increased potential to evade prosecution (motivated offender). A study by Sobsey [
18] supports this proposition and demonstrates the substantial vulnerability of victims with ID, noting that 44 % of perpetrators had contact with a victim through disability services, in which the victim was in close proximity with the perpetrator and was less likely to recognise or report a crime due to the apparent legitimacy of the disability service. As such, victimisation can be seen as a product of complex interactions between the environment, the victim(s) and the perpetrator(s).
The available research is consistent with this theoretical stance, noting increased rates of victimisation among people with ID compared to the general population. Wilson and Brewer [
6] estimated that a diagnosis of ID doubled the risk for victimisation and vastly increased the likelihood of sexual assault and being the victim of robbery compared to the general community. Other studies report similar findings, estimating that the risk of victimisation is between three and seven times higher in people with ID compared to the general community [
11,
13,
19]. Of particular concern are the high rates of sexual victimisation [
19‐
22] and the finding that the majority of crimes were committed multiple times [
23], across a range of ages and environments [
13,
24]. Of note, these victimisation experiences are rarely reported to authorities [
6,
7,
19,
23].
Surprisingly, there is little knowledge on the relationship between gender and victimisation among people with ID. While there is some evidence from general population estimates implicating males as being more vulnerable to victimisation [
25], it is not known whether this trend exists in the ID population. That being said, tentative conclusions can be drawn from general disability research, which suggests that females are more susceptible to violent and sexual victimisation compared to males [
20,
26]. In sum, the available literature suggests that people with ID are at a greater risk of victimisation compared to non-disabled members of the general public, however it is far from a robust conclusion. This is for two reasons; first, of the few studies conducted, too many focus on outdated retrospective self-report data, which rely on the individual recalling events and judging whether a crime was committed. This presents a difficulty with people who have an ID as there are often disruptions in their memory functioning and judgements about their own, and the activities of others, when recalling crimes [
7], consequently past research may either under report or over report actual rates of victimisation. Second, the operational definition of ID varies between studies and importantly, samples tend to be small and lack community comparison groups; both of which would arguably help contextualise the nature, direction and extent of victimisation [
10].
Intellectual disability and offending
Like victimisation, the functional deficits evident in ID suggest that people with ID may also be likely to offend. This sentiment has a long tradition [
27], attracting consistent research attention over the years; with studies claiming that people with ID are overrepresented among individuals processed by the criminal justice system [
28‐
30]. The estimated prevalence of offending in people with intellectual disability ranges from two to ten per cent and varies depending on the population and methods utilised [
27,
31]. There is much variation within prison populations, with estimates ranging from less than 2 % to as high as 30 % [
29,
30], yet there is little agreement on a standardised conceptual definition of what criminal offending is across these studies. A recent systematic review, pooling results from ten studies and including a total of 11,969 prisoners concluded that typically 0.5 % to 1.5 % of prisoners are diagnosed with intellectual disabilities [
32]. Estimating offending prevalence with prison populations is problematic as many individuals with ID have been diverted into the community or forensic services rather than prison, so there may be an under-estimation of the true prevalence using this method.
Court appearances and police contacts provide an alternative means of establishing prevalence and are more sensitive, as these records more adequately capture the extent of contact people have with the criminal justice system. The available literature at this interface estimates that around 1 in 10 people with ID will come into contact with the police or courts as a perpetrator of crime [
14,
33]. These rates are substantially different to those in the general population, with males with ID being three times more likely than males in the general public to have a prior conviction, while females have been found to be four times more likely to have a prior conviction. Interestingly, this figure was more pronounced for violent offences, with males four times higher and females 25 times higher, therefore potentially suggesting a significant vulnerability to violent offending among people with ID [
34].
Some evidence suggests that people with ID are susceptible to the perpetration of specific crimes, such as sexual offences [
2]. Further, there are additional factors that potentially complicate the hypothesised link between ID and offending, with findings revealing that complexities such as childhood neglect, physical health problems, mental health problems and perinatal adversity are particularly common among offenders with ID. There is also some suggestion that offenders with ID may be less effective at evading police and more visible as perpetrators [
5] and this is the reason for increased prevalence rates.
The relationship between crime perpetration and ID and mental illness, which is highly comorbid with ID [
33], has received empirical scrutiny. Hodgins and others [
34] estimated that the presence of mental illness increased offending by five times in psychiatric inpatients compared to those with ID who had not been admitted for mental health treatment. Additionally, Vanny [
33] found that nearly half of those people with ID who were referred to court had a mental illness, thereby suggesting a more complex group who may be at increased risk of criminal offending.
Aims and hypotheses
Against this background, this study sought to determine the prevalence of criminal victimisation and offending in an ID population and to compare this to a sample of people drawn from the general population. Based on the findings of Wilson and Brewer [
6] it was hypothesised that people with ID would have higher rates of victimisation and offending relative to the community sample. Secondly, based on the findings by Sobsey [
19], sexual crimes were expected to be increasingly more common in the ID group. Thirdly, the added complexity brought about by having comorbid mental illness [
34] was hypothesised to increase the risks of victimisation and offending above that observed in people with ID only. Finally, males with ID were hypothesized to be at greater risk of both victimisation and offending than females with ID.