Youth mental health lays the foundation for health and well-being across the life course. Mental health problems, such as psychosomatic complaints, depression, and anxiety, are recognised as significant challenges in young people’s lives that can have both immediate and long-term consequences. Social connectedness, i.e., the extent to which individuals feel belonging and integration within their social relationships, plays an important role in shaping youth mental health. This thesis aims to contribute to an increased understanding of mental health problems across adolescence and young adulthood – life stages characterised by significant social transitions – by examining the development of these problems and exploring their associations with social connectedness. Drawing on repeated survey data and linked register information, the four empirical studies in this thesis investigated common indicators of mental health problems – psychosomatic complaints, depression, and anxiety – and their links with different aspects of social connectedness.

Study I explored the link between psychosomatic complaints during adolescence and depression and anxiety symptoms in young adulthood. The findings revealed graded associations between both the frequency and number of psychosomatic complaints and later depression and anxiety symptoms. Additionally, both earlier and more recent complaints were linked to subsequent depression and anxiety symptoms, while persistent psychosomatic complaints showed stronger associations in girls. This suggests that adolescent psychosomatic complaints may serve as a marker for underlying distress that could develop into depression and anxiety in young adulthood. Therefore, if left unaddressed, frequent, numerous, and persistent psychosomatic complaints in adolescents may contribute to more serious mental health problems over time.

Study II focused on parenting practices and psychosomatic complaints across middle and late adolescence. Although social circles widen during adolescence, to include peers, friends, and teachers, parental relationships remain fundamental. This study examined three core parenting practices – parental support, knowledge, and rule-setting – and demonstrated an especially important role of parental support in mitigating youth psychosomatic complaints. While no evidence was found for a longitudinal impact of parenting practices on later psychosomatic complaints, increases in parental support and knowledge over time were linked to a decrease in youth psychosomatic complaints, with parental support showing the most consistent association. These findings suggest that continuous and increased parental support may be particularly important for youth mental health during these ages.

Study III examined the relationship between belonging, loneliness, and psychosomatic complaints in late adolescence and young adulthood. This study regarded belonging and loneliness as related, yet distinct constructs. Belonging and loneliness were analysed separately and as the cross-combinations of these, in accordance with the dual-continuum model. The findings revealed that belonging was negatively associated and loneliness was positively associated with psychosomatic complaints in cross-sectional analyses. Furthermore, individuals in the so-called socially distressed group – those experiencing high loneliness and low belonging – exhibited both cross-sectional and prospective associations with increased psychosomatic complaints.

Study IV further explored how loneliness was prospectively linked to depression and anxiety in young adults. This study examined the association of loneliness with subsequent self-reported depression and anxiety symptoms, as well as with psychiatric diagnoses based on register data. The findings showed that loneliness in late adolescence was linked to a greater likelihood of both self-reported symptoms and clinical diagnoses of depression and anxiety in young adulthood, even after accounting for sociodemographic characteristics and indicators of prior mental health problems.

This thesis demonstrates the interrelated nature of mental health problems and underscores the importance of social connectedness as a social determinant of health among young people. The individual studies offer empirical evidence on how various aspects of social connectedness – that is, social support, belonging, and loneliness – are linked to mental health problems across adolescence and young adulthood. While high social connectedness can reduce mental health problems, low social connectedness not only contributes to current mental health challenges but also has lasting impacts that can extend into adulthood. Furthermore, various indicators of social connectedness do not exist in isolation and may influence youth mental health both independently and collectively. Overall, this thesis highlights the crucial role that social connectedness plays in youth mental health throughout adolescence and young adulthood.