ABSTRACT
Intellectual disability1 (ID) is a generalised disorder that involves impairments of mental abilities that impact intellectual functioning and adaptive behaviour, such as reasoning, learning and problem-solving skills. There is no specific age requirement, but the onset of symptoms is typically in early childhood. According to the DSM-5, ID equals an IQ score of 70 of below (APA, 2013). Approximately 1 to 2% of the general population has an intellectual disability, whereas prevalence rates among forensic psychiatric patients typically vary between 3 and 50% (e.g. Deb, Thomas, & Bright, 2001a; Holland, 2004). Many people with IDs show high-challenging or aggressive behaviour. Definitions of aggressive behaviour vary, but the most common forms are physical aggression, sexual aggression, self-injurious behaviour, temper tantrums, screaming and shouting (Deb et al., 2001b; Myrbakk & von Tetzchner, 2008). Although the true extent of aggressive behaviour in ID populations is unclear, some studies report prevalence rates ranging from 11 to 60%, which would suggest that the prevalence rates are up to three to five times higher than in non-disabled individuals (Hogue et al., 2006; Janssen, Schuengel, & Stolk, 2002; McClintock, Hall, & Oliver, 2003; Rojahn, Zaja, Turygin, Moore, & van Dingen, 2012; Sturmey, 2002; Taylor, Novaco, Gillmer, Robertson, & Thorne, 2005). Higher rates of aggressive behaviour are found in institutionalised and secure settings. This is not surprising because aggressive behaviour is often the reason why they are institutionalised or admitted to a forensic psychiatric hospital in the first place (Benson & Brooks, 2008; Bhaumik et al., 2009; Embregts, Didden, Schreuder, Huitink, & van Nieuwenhuijzen, 2009). Studies demonstrate that offenders with IDs are more likely to be convicted for sexual offences, arson, theft and burglary, and criminal damage compared to non-ID offenders (Asscher, van der Put, & Stams, 2012; van den Bogaard, Embregts, Hendriks, & Heestermans, 2013; Embregts et al., 2010; Simpson & Hogg, 2001). ID offenders are also responsible for a substantial amount of aggressive incidents within forensic settings. This behaviour can interfere with participating in social activities and can lead to social rejection; it can also impair new learning skills, cause physical injury to self and others and can incur great costs (Novaco & Taylor, 2008; Travis & Sturmey, 2013). In general,
crime and violence have an enormous impact on society in terms of damage to persons and property, the effect on the quality of life of victims and their families, as well as the costs of prosecuting offenders and detaining them for prolonged periods of time.
