55 Behavioral problems in Take it Personal! 3 1. Introduction Problematic alcohol, cannabis, and other illicit drug use among adolescents and young adults with mild intellectual disability or borderline intellectual functioning increasingly receives attention in clinical practice and research (Chapman & Wu, 2012; Didden et al., 2020). A mild intellectual disability is a disability that manifests during childhood. It is characterized by an intelligence quotient (IQ) score between 50-70, and limitations in adaptive behavior impeding a range of everyday social and practical skills (American Psychiatric Association, 2013). The DSM-5 describes borderline intellectual functioning as a condition for people whose limited intellectual functioning is the focus of- or impacts their treatment (American Psychiatric Association, 2013). This diagnosis is typically given when IQ is roughly between 70 and 85 (Wieland & Zitman, 2016). Similar to those with a mild intellectual disability, people with borderline intellectual functioning often lead problematic lives, facing e.g. social and coping difficulties, and are vulnerable for developing psychopathologies such as substance use disorder (Emerson, 2011). Slayter (2010) found that 2.7% of youth with a mild intellectual disability1 had a disorder for psychoactive substances, such as alcohol, cannabis, or other drugs. Although this prevalence rate is lower than that of youth without a mild intellectual disability (5.2%), the relative risk of developing a substance use disorder after initiating substance use is considered to be higher in youth with a mild intellectual disability (Didden et al., 2020; van Duijvenbode & VanDerNagel, 2019). This is because abstinence rates are proportionally higher in youth with a mild intellectual disability diagnosis compared to those without that diagnosis. Moreover, prevalence rates of individuals with a mild intellectual disability in substance use disorder samples of 30% (Braatveit et al., 2018) and 39% (Luteijn et al., 2017) indicate a clear overrepresentation of this target group in addiction care (van Duijvenbode & VanDerNagel, 2019). Although many individual-specific factors contribute to developing a substance use disorder, the impairments in cognitive, social, and coping skills that are characteristic of this target group (American Psychiatric Association, 2013) unequivocally underlie the elevated risk of substance use disorder in people with a mild intellectual disability (Chapman & Wu, 2012). The increased urgency to tackle substance use disorder in this target group is reflected by a growing evidence base of intervention programs for problematic substance use in the mild intellectual disability population (van Duijvenbode & 1 To avoid an excess of the lengthy term ‘mild intellectual disability or borderline intellectual functioning’, the target group is referred to as ‘mild intellectual disability’ throughout this chapter. This is done to improve readability, but please bear in mind that, formally speaking, those with borderline intellectual functioning are included throughout.
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