Compared with PRT, ACT resulted in a reduction in the NTS total score in patients with a history of suicide attempt in the past year, independent of the improvement in depressive symptomatology. The Cyberball game elicits painful feelings due to social exclusion (i.e., social pain), which shares features with physical pain [27]. Our finding is particularly compelling because it underscores the capacity of ACT to address social perceptions, in addition to its effect on depressive symptoms. The reduction in the perception of social exclusion is noteworthy, given the established link between ostracism and a heightened risk of suicidal behaviors [28]. It also aligns with the temporal model of ostracism effects proposed by Williams [29]. According to this model, interventions such as ACT prevent the intense psychological distress associated with the resignation stage by promoting adaptive coping strategies during the previous reflexive and reflective stages of the reaction to ostracism.

Moreover, the feeling of being ignored, which implies the interpretation of negative intent from others toward oneself, was significantly reduced by ACT but not PRT. ACT may help prevent the reflexive stage of ostracism by mitigating the perception of being ignored. Indeed, ACT may equip individuals with the resilience needed for not interpreting a lack of ball tossing as a personal disregard, thus avoiding the initial pain response described by Williams.

The NTS total score changed significantly from before to after the intervention, with different trajectories in the ACT and PRT groups. By fostering increased psychological flexibility, ACT encourages a more adaptative relationship with one's thoughts and feelings [30], thereby mitigating the threat of ostracism to fundamental needs. This effect was particularly significant for the belonging subscore, reinforcing the pivotal role of social connections in the management of suicidal patients. Indeed, low belongingness is at the core of the interpersonal theory of suicide [31, 32]. The improved belongingness through the ACT offers perspectives for interventions, highlighting the ability of this therapy to target and improve critical dimensions that contribute to the suicidal process.

We found that suicidal ideation severity and the perception of social exclusion decreased concomitantly. However, the correlation between the increase in the NTS total score (reduction in social distress) and the reduction in suicidal ideation severity (C-SSRS score) was not significant, probably due to a ceiling effect. Indeed, only 11 patients (34%) reported active suicidal ideation, limiting the possibility of achieving statistical significance. Nevertheless, this observation strengthens the importance of reducing social distress in suicidal patients. Moreover, we found that social distress progressively decreased with changes in therapeutic processes, including psychological flexibility, cognitive defusion, and mindfulness. It emphasizes the pivotal role of ACT in bolstering reflective coping strategies [29]. This form of adaptive coping is instrumental for potentially tharting the progression to the resignation stage, where enduring psychological distress becomes a significant concern. Notably, such enduring distress is exacerbated by pain catastrophizing, a cycle of rumination that amplifies the perception and impact of pain. Pain catastrophizing, marked by magnification, rumination and a pervasive sense of helplessness about pain, considerably increases the intensity and persistence of psychological distress. A recent study reported that in patients with low back pain, pain catastrophizing is an independent risk factor for progression to chronic widespread pain [33]. Moreover, pain catastrophizing is enhanced by experiential avoidance (an attempt to evade or escape unwanted internal experiences, including thoughts, feelings, and bodily sensations), which is targeted by the ACT. The relationship between fear-avoidance beliefs and pain intensity indicates a reciprocal association: patients with increased fear avoidance have greater pain intensity, which in turn induces greater fear avoidance [34]. Moreover, a comprehensive meta-analysis [35] revealed moderate to strong associations between experiential avoidance and suicidal behaviors. By fostering psychological flexibility, mindfulness, and cognitive defusion, ACT directly counters the detrimental cycles of experiential avoidance, pain catastrophizing, and fear avoidance and decreases painful feelings related to the social exclusion and suicidal dimensions.

ACT, as a third-wave cognitive‒behavioral therapy, emphasizes acceptance and psychological flexibility, which are crucial for navigating through disturbing thoughts and experiences. This approach diverges from attempts to control or avoid these experiences and facilitates a profound change in the individual's relationship with distressing thoughts and sensations [36]. By decreasing rumination, ACT facilitates a shift toward increased resilience and meaningful life engagement. This enhancement of psychological processes through ACT highlights a shift toward a resilient self-concept anchored in a meaningful life and counteracts the increased implicit associations of "death" and "me", relative to "life" and "me", caused by ostracism [37]. As self-concept is at the root of all our experiences [38], understanding the processes underlying self-concept changes, at the core of recent developments in relational frame theory [39], is a major concern in psychotherapy.

Our results are in line with those of a previous study showing that dispositional mindfulness, a key aspect of ACT, is negatively associated with the perception of exclusion and ruminations following experimental exclusion [40]. They also provide insights into the dynamic nature of this association through the prospective design of data collection, which highlights the transformative potential of mindfulness and acceptance in addressing social pain. By emphasizing psychological flexibility and acceptance, ACT helps individuals reinterpret experiences of social exclusion, alleviates immediate distress, and promotes long-term well-being and resilience despite ostracism.

For this study, patients with current suicidal behavior disorders were included because the risk of reattempts is highest in the first year following a suicide attempt. ACT may be initiated during this critical period because, in these patients, it reduces suicidal ideation and depression [16] as well as perceived ostracism, which may amplify interpersonal difficulties and barriers to adherence to care. The decreased feeling of social exclusion may contribute to reducing suicidal ideation, restoring social connections, reducing loneliness, and improving adherence to care.

Our study has several limitations. The sample was small and included mainly women who feel more excluded and ignored in reaction to social exclusion [41]. This may limit the generalizability of our results. The Cyberball game has never been repeated in previous studies, raising the question of a learning effect. However, the absence of a significant reduction in the posttherapy NTS total score in the control group (PRT) and the realistic estimation of ball receipts indicate task validity even the second time.

The findings of this study pave the way for new research directions on the application of ACT in reducing social distress among suicidal patients. A valuable area of future research could explore whether ACT might also reduce perceived ostracism in other at-risk populations, such as adolescents or individuals with personality disorders. Future research could also investigate the underlying mechanisms through which ACT reduces social distress, particularly by examining neurobiological features using functional MRI or peripheral measures (for example inflammatory or epigenetic markers).

In conclusion, ACT offers a significant therapeutic advantage to individuals who experience perceived ostracism, particularly those with a history of suicide attempt. Through psychological flexibility, mindfulness, and cognitive defusion, ACT reduces immediate social pain and may promote long-term resilience. The transformative potential of ACT, in redefining the relationship with distressing thoughts and experiences, underscores the critical role of relational frame theory advances in the future of psychotherapy to address the complex interplay of self-concept, social pain and suicidality. The significant reduction in perceived social exclusion achieved through ACT in our study suggests that this approach could be integrated more broadly into therapeutic protocols for patients at suicide risk, particularly in the early stages of post-attempt care when the risk of recurrence is high.