Abstract
BACKGROUND AND AIMS: Adolescent mental health has declined in recent decades. Few studies have explored whether shifts in interpersonal behaviours- both in-person and online- can help explain this decline. Using data from two Swedish cohorts (2007-2008, 2023-2024), we examined (1) 15-year time trends (2007-2008 vs. 2023-2024) in interpersonal behaviours (victimization, aggression, prosocial behaviours) and mental health (externalizing/internalizing problems); (2) one-year bidirectional associations between them; (3) the unique role of cybervictimization/cyberaggression in relation to mental health; and (4) adolescents' accounts of online experiences that made them 'feel bad'. METHODS: Two longitudinal school cohorts of Swedish adolescents (2007-2008: N = 911-987, M(age) = 13.71-14.76 years; 2023-2024: N = 768-806, M(age) = 13.89-14.89 years) completed self-report measures of interpersonal behaviours and mental health. In 2023-2024, a subset of adolescents (n = 127, 16.5%) answered open-ended questions about negative online experiences. RESULTS: From 2007 to 2008 to 2023-2024, prosocial behaviours moderately declined for girls and boys, while internalizing problems increased moderately for girls and weakly for boys. Meanwhile, victimization, aggression, and externalizing problems increased at a weak-to-moderate level among girls. Victimization and being treated well by others showed moderate bidirectional relationships with mental health. Cybervictimization/cyberaggression had weaker associations with mental health than did in-person behaviours. According to the qualitative analysis of negative online experiences, adolescents reported harassment, social exclusion, perceived standards/expectations, time-consuming activities, and exposure to distressing content. CONCLUSIONS: Deteriorating adolescent mental health over the past 15 years has occurred alongside rising interpersonal difficulties. The findings further suggest a reciprocal relationship between social challenges and mental health, with hostile online environments amplifying- but not primarily driving- these issues. A holistic perspective that accounts for both in-person and online experiences is essential to better understand and support adolescent well-being.