Self-harming behaviors can present in a variety of forms. One such form involves more direct self-harm through nonsuicidal self-injury (NSSI), which is defined as the intentional destruction of a person's own body tissue without suicidal intent (Nock and Favazza, 2009). Other behaviors, such as disordered eating behaviors, which include restrictive eating (i.e., intentionally eating less than is appropriate for context or nutritional needs), have been conceptualized as indirect forms of self-harm that cause cumulative damage to the body (Claes and Muehlenkamp, 2014). Unsurprisingly, NSSI and disordered eating behaviors are highly comorbid, with approximately 27.3% of eating disorder patients engaging in NSSI (Cucchi et al., 2016), and up to 80% of individuals engaging in NSSI exhibiting disordered eating (Turner et al., 2016; Yiu et al., 2015). Although eating disorders and NSSI are associated with other comorbidities (e.g., 65% of individuals with an eating disorder have a comorbid anxiety disorder (Swinbourne et al., 2012); 24% of individuals engaging in NSSI have a substance use disorder (Liu, 2023)), the co-occurrence of restrictive eating and NSSI presents a significant concern, as each of these forms of self-harm is closely associated with suicide risk (Brausch and Perkins, 2018; Turner et al., 2015).

Historically, much of the research on the co-occurrence of NSSI and disordered eating has focused on the comorbidity of NSSI with binge eating and purging (Kiekens and Claes, 2020), given the seemingly similar presentations as under-controlled behaviors. However, recent studies show that restrictive eating is associated with NSSI beyond binge eating and purging, indicating a unique role of restrictive eating in the co-occurrence of disordered eating and NSSI (Wang et al., 2018, 2020). Indeed, when explicitly questioned about their intentions for engaging in these behaviors, restrictive eating and NSSI had remarkable similarities, being more closely related to self-destructive motivations than binge eating or purging (Fox et al., 2019; Kline et al., 2023). Moreover, restrictive eating and NSSI are distinguished from other problematic behaviors by their non-hedonic nature, being associated with typically unpleasant and non-rewarding experiences, such as hunger or physical pain, compared to behaviors such as binge eating and substance use, which are more clearly connected to the pursuit of primary reinforcers (Haynos et al., 2023a, Haynos et al., 2023b).

In this context, while earlier studies have commonly linked disordered eating and NSSI to impulsivity (Claes et al., 2013; Paul et al., 2002), this association may be influenced by studies on binge eating and purging and not be fully applicable to restrictive eating and NSSI. Restrictive eating and NSSI are often goal-directed and/or planned behaviors aimed at seeking automatic positive functions (Selby et al., 2014; Wang et al., 2021), which may be driven by feelings of anhedonia and depressive symptoms (Franko et al., 2004; Lian et al., 2017; Miotto and Preti, 2007). Indeed, both restrictive eating and NSSI have been associated with heightened responsivity to disorder-related cues but reduced reward responsiveness to general cues (Case et al., 2021; Haynos et al., 2020; Osuch et al., 2014). Therefore, reduced reward responsivity could be a potential risk factor in the co-occurrence of restrictive eating and NSSI, prompting individuals to seek out multiple behaviors that elicit rewarding sensations to reduce anhedonia. Further, emerging evidence suggests that restrictive eating may involve another aspect of reward responsivity, habit formation, wherein an initially rewarding behavior eventually becomes an over-trained habit that is difficult to change (Uniacke et al., 2018). Although habit formation has not been commonly investigated in NSSI, evidence suggests that some direct self-harming behaviors, such as body-focused repetitive behaviors (i.e., hair-pulling, skin-picking), may be maintained by habit processes (Skurya et al., 2020). Thus, those with increased propensity to form habits may be more vulnerable to developing multiple forms of self-harm.

Thus, we hypothesized that dysfunctions in reward and habit processes may be risk factors associated with the co-occurrence of restrictive eating and NSSI. Specifically, we expected that 1) lower reward responsivity and heightened habit formation would be linked to an elevated risk of the co-occurrence of restrictive eating and NSSI; 2) this association would be maintained after controlling for a specific facet of impulsivity (i.e., lack of planning/thinking ahead), a construct that has previously been linked to disordered eating and NSSI; and 3) lower reward responsivity and higher habit formation would be associated with the severity of NSSI and eating disorder pathology.