In this study, adolescents reporting loneliness and low resilience showed an increased risk of anxiety and depression symptoms in young adulthood after an 11-year follow-up. The combination of loneliness and low resilience was synergistic and further elevated this risk in young adulthood. These findings align with previous research linking adolescent loneliness to higher anxiety and depression risk across all ages. For instance, Stickley et al. (26) reported higher odds of headaches, anxiety, and depressive symptoms among lonely adolescents. Moreover, a multilevel meta-analysis by Maes et al. (74) found a positive association between loneliness and social anxiety symptoms across childhood and adolescence. Additionally, Beutel et al. (12) highlighted a strong association between loneliness and generalized anxiety and depression, especially among middle and late adulthood individuals. Our findings also corroborate with prospective studies done by Kehusmaa et al. (29) and Xerxa et al. (30), showing that negative factors in childhood and adolescent social relationships, particularly loneliness, are associated with anxiety and depression in young adulthood. Moreover, supporting evidence for the sensitivity analyses of the present study comes from studies investigating healthcare utilization patterns among lonely individuals. For example, Beutel et al. (12) noted that lonely individuals were more likely to seek medical consultation and inpatient treatments for anxiety and depression. Similarly, Junker et al. (75) showed that loneliness, often accompanied by anxiety and depression, increased the risk of subsequent self-harm hospitalization among Norwegian adolescents. Moreover, Modin et al. concluded that (76) low childhood social status in the school class among girls, which indirectly mirrored their loneliness, predicted an increased risk of them becoming hospitalized with anxiety and/or depression at long-term.
The pathway between loneliness and anxiety and depression is multifaceted, encompassing psychological, biological, and social factors (77, 78). Loneliness can lead to negative cognitive patterns, such as rumination and self-criticism, which are linked to anxiety and depression (14, 79). Moreover, the chronic stress associated with loneliness can dysregulate biological systems, including the hypothalamic-pituitary-adrenal (HPA) axis and the immune system, which play pivotal roles in the development and maintenance of anxiety and depression (80–82). Loneliness affects youth's diurnal cortisol, a key stress hormone (83, 84), potentially causing chronic increases in cortisol in young adulthood, leading to higher risks of depression, anxiety, inflammation, and other adverse health outcomes (85–87). Adolescents’ heightened exposure to loneliness can be explained by their strive for independence from their families and seeking social and emotional connections with their peers (23). Failure to form such intimate relationships during this developmental stage can increase the risk of experiencing loneliness. (24, 25). Chronic loneliness, during adolescence, can hinder social skill development, leading to low self-esteem, and vulnerability and thus exacerbating anxiety and depression (88). Moreover, loneliness disrupts cognitive processes, increasing susceptibility to anxiety and depression (89). Lonelier individuals may exhibit disrupted emotional regulation, leading to increased negative emotions and a higher propensity for risky health behaviors, which could impact future mental health outcomes (90). Additionally, lonelier individuals experience poorer sleep quality in young adulthood, potentially worsening their long-term mental health (91).
The current study also investigates the role of low resilience as an additional risk factor in the pathway between adolescent loneliness anxiety, and depression in young adulthood. Several cross-sectional studies performed in different age groups have focused on the protective role of resilience. For example, a study conducted by Zhao et al. found resilience mediated the link between loneliness and depression among the elderly, while studies on working-age adults and Norwegian adolescents showed resilience moderated this relationship (92–94). In contrast to the protective effect, the detrimental effect of low resilience was shown by Fangyan et al. (95). They compared depression levels among young adults with high and low resilience, showing that higher loneliness correlated with increased depression among those with low resilience, likely due to stress hormones like cortisol, which can lead to long-term neurological and physiological issues (96). Moreover, individuals with low psychological resilience seem to have compromised control of brain circuits involved in emotion and fear (97) and poor cortico-limbic inhibition, representing a dysregulated emotional response to stress that may result in elevated cortisol exposure (98). Likewise, loneliness and social isolation during adolescence precipitate a state of vulnerability or social frailty (80), rendering individuals more susceptible to adverse outcomes. This vulnerability may arise from a lack of protective factors like familial, social, or communal support, which is crucial for managing life's pressures. Consequently, loneliness and isolation can diminish resilience, leaving individuals more vulnerable to anxiety and depression (99). Our study extends prior research with an extensive prospective cohort and thorough 11-year follow-up, adjusting for a wide range of covariates. We found that adolescent loneliness combined with low resilience significantly increased the risk of anxiety and depression in adulthood, suggesting a synergistic effect not explained by either factor alone, as indicated by the RERI. This interaction may stem from compromised stress coping mechanisms in lonely individuals with low resilience, impacting social interactions and exacerbating feelings of loneliness. Further research is needed to confirm causal relationships and explore additional factors influencing the complex interplay between loneliness, resilience, and long-term mental health outcomes.
4.1. Strengths and Limitations
Our study's strengths include a prospective design that establishes a clear temporal sequence, a large sample size at baseline and 11-year follow-up, and the ability to assess the interaction between adolescent loneliness and low resilience on young adult anxiety and depression. Additionally, the comprehensive dataset allowed control for various potential confounders. Resilience among adolescents was measured by the validated READ scale, covering relevant protective elements within resilience (54). Similarly, anxiety and depression among young adults were measured by using the well-validated HADS questionnaire (61). We conducted sensitivity analyses to ensure robustness, using an alternative definition of anxiety and depression that included those on antidepressant medication and those with recent healthcare visits, which increased precision and avoided misclassification. Additionally, we repeated all analyses after excluding adolescents who reported anxiety and depression at baseline.
This study has limitations. Self-reported measures may introduce biases such as social desirability and underreporting, while non-participation and absenteeism on data collection days may cause selection bias, potentially underestimating loneliness and its link to anxiety and depression. Despite an 11-year follow-up, more frequent assessments could better capture changes in loneliness and resilience. Self-reported loneliness is also prone to underreporting due to social stigma (100). Loneliness is complex and ideally should be measured with a multi-item scale to account for variations in intensity, circumstances, and time with direct and indirect questions. Nevertheless, self-reported measures of loneliness have been shown to be equally reliable and valid as other forms of assessment (101). Another issue is the categorization and applied dichotomization of loneliness and resilience. However, dichotomized outcomes may be necessary to communicate a comparison of risks (102). Moreover, it is reassuring that this same dichotomization has been applied in earlier research among adolescents (50, 51, 103). Moreover, despite the broad ranking and categorization of loneliness in this study, it fails to distinguish between adolescents who are rarely lonely and those who are never lonely. Another limitation is residual confounding despite controlling for relevant factors. Furthermore, it is uncertain if loneliness, low resilience, anxiety, and depression at baseline mutually influenced each other and thus increased the risk of anxiety and depression prospectively. Nevertheless, we excluded the adolescents who reported a score of ≥ 2 on the HSCL-5 at baseline in the sensitivity analyses to limit the possibility of reverse causation. In the current study, the HADS was used as a screening tool to capture symptoms of anxiety or depression and not a clinical diagnosis (104). Although we strengthened the definition of anxiety and depression by adding medication use and hospital admission, we cannot rule out possible misclassification bias, as we do not have any objective tests performed. Thus, the limitations in identifying clinical anxiety or depression in adolescence and young adulthood, respectively, may limit the generalizability of our results.