156 Chapter 7 change. Chapters 5 and 6 then described idiographic avenues for studying how behaviors change. With each chapter, the focus of the lens was changed to one with a narrower field of view that provides more detail on individual-level change processes. Is a wider or narrower lens more preferable? My overall conclusion is that all lenses contain value for understanding behavioral change. Even though the lenses are different, they can, and should, be integrated. That is, scientists and practitioners in the intellectual disability field should adopt theory and methodology that prioritize understanding change idiographically before making nomothetic claims. Below I explicate the lessons learned and suggestions for future research in four parts: theory, study design, analysis, and clinical practice. Theory Nomothetic theory behind the theory Research and practice are inherently theory-laden. For instance, “Take it Personal!” targets four personality-risk profiles because personality is theorized to be a key factor in substance use (Poelen et al., 2017; Woicik et al., 2009). Additionally, emotional and behavioral problems were theorized to be interrelated with both personality traits and substance use in individuals with mild intellectual disability (Chapman & Wu, 2012; Taggart et al., 2006). The explicit theory guiding Chapters 2 and 3 was thus that adequately targeting personality traits in treatment would lead to changes in substance use and other behavioral problems (cf. Schijven et al., 2020). Importantly, there was also a crucial implicit theory guiding the nomothetic chapters: a theory behind the theory. The title of Chapter 3 hints at this. “Exploring the role of emotional and behavioral problems . . .” suggests that there are components (e.g., emotional and behavioral problems) whose roles within a broader mechanism (e.g., personality → substance use) could be explored. We investigated whether, for example, there are group differences over time for withdrawal problems or whether rule-breaking moderates the intervention effectiveness on substance use. That entails pointing to an input (an independent variable; called as such because its value should not be dependent on any other values; Field, 2017) that possibly changed an output (the dependent variable). How these variables influence each other remains vague, but statistical models may give the impression that this happens in a linear billiard-ball-like fashion (cf. Heino et al., 2021). The underlying theory was that unique independent components (e.g. emotional and behavioral problems, personality) might explain a unique proportion of the variance in other components (substance use). In other words, we considered it worthwhile to focus on specific variables to study mechanisms of change.
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