Self-injurious thoughts and behaviors, including suicide ideation, suicide attempt and non-suicidal self-injury (NSSI) have become a public health concern worldwide (Wen et al., 2023). In community samples, the lifetime prevalence of suicide ideation, suicide attempts and NSSI were 15.1 %, 2.6 % and 6.2 % respectively (Liu et al., 2022). Suicide ideation was associated with suicide attempts and death by suicide, and the history of suicide attempt was known to be the strongest clinical predictor for death by suicide (Fazel and Runeson, 2020). NSSI, referring as direct and deliberate destruction of one’ body tissue in ways that are not socially or culturally sanctioned and without suicidal intention (De Luca et al., 2022), is also strongly associated with suicide (Geulayov et al., 2019). Thus, understanding the sociodemographic risk factors and triggers of self-injurious thoughts and behaviors could provide effective perspectives to prevent suicide.

One particularly predisposing factor for self-injurious thoughts and behaviors was childhood trauma (Fazel and Runeson, 2020), referring as adverse exposure to one or more events (abuse, neglect and other exploitations) beyond one's coping ability in childhood (Wang et al., 2022). It's a complex concept with varieties of classifications, among which physical neglect, emotional neglect, physical abuse, emotional abuse and sexual abuse were the five most recognized types (Cai et al., 2023). Although prevalence of childhood trauma varied across different populations, trauma form and measuring methods (e.g., 9.6 % ∼ 17.6 % in the United Kingdom (Office for National Statistics, 2020) and 11.6 % ∼ 18 % in the United States (Merrick et al., 2018)), it has raised global concerns regarding the impact on both physical and mental health. Exposure to trauma as a child may hamper development of socioemotional and interpersonal skills in adulthood through dysregulation of emotional response (Poole et al., 2018). Existing literatures suggested that adult survivors of childhood trauma tended to have negative attitudes towards other people, avoid building comprehensive relationship with others and less likely to seek support from peers, leading to higher risk of social isolation and loneliness (Allen et al., 2023; Barton et al., 2023; Reinhard et al., 2022).

Social isolation was an objective portrayal of a social situation described by poor social ties, while loneliness referred as the subjective aspect of social condition of isolation (Motillon-Toudic et al., 2022). Social support, especially emotional support reflected one's social connectedness with family, friends, and the community (Margolis et al., 2022). Combining together, social isolation, loneliness and emotional support could represent individuals' social health (Manera et al., 2022; Margolis et al., 2022), which was strongly correlated with self-injurious thoughts and behaviors. Social isolation, loneliness, alongside with lack of emotional support (Solomonov et al., 2023), had positive and direct relationship with suicide and self-harm (Blázquez-Fernández et al., 2023; McClelland et al., 2023). In accordance with interpersonal theory of suicide (Van Orden et al., 2010), the most near mental states preceding the suicide ideation were the lack of belongingness feeling (self-reported loneliness, living alone, fewer friends, non-intact family, social withdrawal and family conflict) (Brooks et al., 2023) and the burdensomeness and liability to others (Duffy et al., 2020), while childhood trauma and mental disorders were at a relatively more distant point in the causal chain of self-injurious thoughts and behaviors. Empirical study suggested indifference and loneliness could mediate the relationship between childhood trauma and suicide ideation in adults, while receiving recognition and support from groups could be possible to intervene this process (Wang et al., 2022). But it's remained unknown whether the relationships between social health and self-harm or suicide differ in adults with or without childhood trauma.

Narrative reviews showed that the effects of social health on self-injurious thoughts and behaviors may vary significantly across sociodemographic groups (Calati et al., 2019; Motillon-Toudic et al., 2022). In A 50-state survey study of thoughts of suicide and social isolation among older adults in the United States (Solomonov et al., 2023), suicide thoughts were associated with lack of social support instead of living alone, and this association were stronger among younger elderlies, those with higher incomes and living in large households. Another prospective cohort study of 43,015 participants showed that social support appeared to be associated with lower risk of suicide death only in individuals with moderate or severe psychological distress (Otsuka et al., 2022). In the fully-adjusted Cox regression models of admitted for self-harm in a UK Biobank study (Shaw et al., 2021), living alone and lack of emotional support was strong risk factors for self-harm hospital admission in male instead of female, while loneliness was prominent risk factor of hospital admissions for self-harm in both gender. Besides, individuals aged over 70 and youngsters aged 15–29 were two special groups of people, for whom social isolation seemed to play a vital role in self-injurious thoughts and behaviors compared to others (Motillon-Toudic et al., 2022). Moreover, social isolation also tended to have stronger effect for self-injurious thoughts and behaviors in sex minorities such as lesbian, gay or bisexual, and people who were passively isolated due to unemployment, rural areas or other issues (Motillon-Toudic et al., 2022). But major impact of loneliness on both suicide ideation and suicide attempt were much more consistent transculturally (Calati et al., 2019). Nevertheless, there was limited knowledge regarding the differences between individuals with and without childhood trauma with respect to the strength of associations between social health and self-injurious thoughts and behaviors, especially NSSI in adults. Based on previous researches (Angelakis and Gooding, 2022; Otsuka et al., 2022; Solomonov et al., 2023), we assumed that in individuals with childhood trauma, social isolation and loneliness may have stronger positive relationship with self-injurious thoughts and behaviors, while emotional support may have a stronger negative association. Meanwhile, as emotional support was also known as a protective factor that could buffer the associations between mental health problem and self-injurious thoughts and behaviors (Fulginiti et al., 2022; Lapane et al., 2022), the interactions within three variables of social health will also be addressed in this study.

To test our hypothesis, this study will use the population-based extensive dataset from the UK Biobank cohort. The UK Biobank cohort included over 0.5 million participants aged over 40 years old at baseline and follows them up continuously (Sudlow et al., 2015). Although designed to investigate the biological basis of health, it also provided enriching phenotypic data and description of childhood exposures and adult mental health variables relevant to our aims (Mutz et al., 2021). Baseline questionnaires includes comprehensive questions about demographic data, physical and mental health data, social isolation, loneliness and emotional support, while online follow-up questionnaires contain details of re-called childhood trauma experience (<16 years old), and evaluation of self-injurious thoughts and behaviors. Therefore, this current study will use UK Biobank data to explore (1) the differences of associations between social health and self-injurious thoughts and behaviors caused by childhood trauma; and (2) the buffering role that emotional support may play in these associations.