Suicide is a significant public health concern globally, defined as the intentional act of ending one's own life (World Health Organization, 2019). Its prevalence varies globally, with the highest rates in low- and middle-income countries, where 77 % of suicides occur (Naghavi, 2019; Turecki et al., 2019). According to the World Health Organization, nearly 800,000 individuals die by suicide each year, equivalent to 1 in every 100 deaths, surpassing fatalities from HIV, malaria, breast cancer, war, and homicide (World Health Organization, 2019). Suicide not only results in the tragic loss of young lives but also imposes substantial psychological, emotional, and economic burdens on families, communities, and society (Ho and Chen, 2023; Turecki et al., 2019). Thus, investigating the etiological factors and mechanisms underlying suicidal risk is imperative.

Over the past several decades, researchers have extensively examined numerous risk factors for suicide, providing critical insights for prevention strategies (Franklin et al., 2017). A key risk factor is non-suicidal self-injury (NSSI), defined as deliberate, direct harm to oneself without suicidal intent (Nock, 2010). Many studies have shown that NSSI is one of the strongest predictors of future suicidal risk (Kiekens et al., 2018; Ribeiro et al., 2016), with its predictive power even surpassing an individual's past history of suicidal risk (Asarnow et al., 2011). However, it remains unclear whether exposure to NSSI behaviors exerts a similar effect. NSSI exposure refers to experiences such as witnessing someone else engaging in NSSI, hearing descriptions of it, or viewing NSSI-related content (Muehlenkamp et al., 2008). This exposure typically occurs in two forms: online self-injury content and witnessing self-injury behaviors among contacts (Zelkowitz et al., 2017).

The prevalence of NSSI exposure is remarkably high. Muehlenkamp et al. (2008) found that 64 % of participants knew someone who engaged in NSSI behavior. Zelkowitz et al. (2017) reported that moderate exposure to NSSI information in news and television was reported by 60 % and 64 % of participants, respectively, with an even higher rate of exposure in movies at 71 %. Compared to direct engagement in NSSI, exposure to NSSI is an indirect factor that may influence suicide risk through various mechanisms. Investigating interaction patterns among NSSI exposure, NSSI behaviors, and suicidal risk could provide valuable insights into their distinct and interrelated effects. We posit that the impact of NSSI exposure on future suicidal risk can be classified into the following patterns.

Exposure to NSSI may increase an individual's likelihood of engaging in NSSI behavior, which subsequently raises the risk of future suicidal risks. This relationship is supported by theoretical frameworks such as the Interpersonal Theory of Suicide (IPTS; Van Orden et al., 2010), the Integrated Motivational-Volitional Model (IMV; O' Connor and Kirtley, 2018), and the Three-Step Theory (3ST; Klonsky and May, 2015). These theories suggest that individuals who engage in self-inflicted violence, such as NSSI, may develop increased tolerance and habituation to physical pain, diminishing their fear of and resistance to suicidal risk (Cleare et al., 2021; Ren et al., 2019).

The social hypothesis of NSSI posits that one's decision to engage in NSSI is often influenced by observing such behaviors in others. Hooley et al. (2018) note in their Benefits and Barriers Model of NSSI that a lack of awareness of NSSI is a significant barrier preventing engagement. Exposure to NSSI—whether through learning about it online or directly witnessing others' behaviors—increases awareness, making self-injury a more viable behavioral option.

Studies have shown significant correlations between NSSI exposure and engagement. Radovic and Hasking (2013) found that exposure to NSSI references in movies, coupled with character identification, was associated with a higher likelihood of engaging in NSSI. Zhu et al. (2016) found that hospitalized adolescents who frequently encountered NSSI in media were more likely to engage in NSSI. Lee et al. (2023) demonstrated a significant rise in emergency department visits for self-harm following NSSI exposure, particularly among adolescents. Real-life interactions among peers also exhibit contagion effects. Syed et al. (2020) noted that awareness of friends' self-injury was significantly related to self-reported NSSI, suicidal ideation, and suicide attempts. Zelkowitz et al. (2017) found significant associations between interpersonal and media exposure to NSSI and a history of NSSI. Based on this evidence, we propose the first relational model: NSSI exposure affects future suicidal risk through the mediating role of NSSI engagement.

In addition to the mediated pathway, NSSI exposure may directly influence suicidal risk independently of NSSI engagement. For example, exposure to NSSI might alter an individual's perception of pain, leading them to underestimate its severity. Dawkins et al. (2021) found that adolescents aware of their parents' NSSI behaviors had lower pain expectations, suggesting that exposure can desensitize individuals to pain, increasing their capacity for suicide. Additionally, frequent exposure to NSSI information may diminish respect for life and reduce fear of death. Gauthier et al. (2018) and Stanley et al. (2020) noted that exposure to painful and provocative events, including NSSI, was associated with fearlessness about death. This normalization of self-injury and death may make suicide appear more acceptable.

Furthermore, exposure to NSSI behaviors—particularly among close friends—can initiate co-rumination, increasing negative emotions and raising suicide risk. Studies highlight the role of co-rumination in the socialization of NSSI behaviors (Giletta et al., 2013; Schwartz-Mette and Lawrence, 2019). When individuals interact with friends who exhibit NSSI symptoms, they may resonate emotionally with their struggles, resulting in shared pain and distress. These mechanisms suggest that NSSI exposure can directly predict future suicidal risk, even without NSSI engagement. Thus, we propose the second relational pattern: NSSI exposure alone significantly impacts the risk of future suicidal risk.

NSSI exposure may also interact with NSSI behaviors to jointly influence suicidal risk. For individuals already engaged in NSSI, exposure to NSSI behaviors further elevates their risk of suicide. Frequent exposure may normalize NSSI, reducing anticipated negative consequences (Cavazos-Rehg et al., 2017) and diminishing aversion to pain and death (Dyson et al., 2016). This desensitization lowers psychological barriers, increasing the likelihood of escalation to more lethal self-harm.

Peers play a crucial role in reinforcing NSSI behaviors (Conigliaro and Ward-Ciesielski, 2023). Exposure within peer groups can create a supportive environment for self-injury, where individuals receive reinforcement for their actions, sustaining and escalating NSSI (Crudgington et al., 2023). Pitman et al. (2023) found that stimulated exposure to peer NSSI significantly reduced participants' ability to resist NSSI impulses, indicating that exposure may be linked to the recurrence or escalation of NSSI behaviors.

Thus, NSSI exposure not only directly influences suicidal risk but also exacerbates the harmful impact of NSSI engagement. We propose the third relational pattern: NSSI exposure positively moderates the association between NSSI engagement and suicidal risk.

To explore the temporal sequence between NSSI exposure and suicidal risk, we employed a short-term longitudinal design with a three-month interval. This interval was chosen to capture immediate effects of NSSI exposure while minimizing attrition. We controlled for depressive symptoms, a well-established risk factor for both NSSI and suicidal risk, to clarify the distinct contributions of NSSI exposure and engagement to suicide risk. This design offers preliminary insights into how NSSI exposure and engagement interact to influence future suicidal risk.