Social connection and its prospective association with adolescent internalising and externalising symptoms: an exploratory cross-country study using retrospective harmonisation

Background: Social connection factors play a key role for young people's mental health. It is important to understand how their influence may vary across contexts. We investigated structural (e.g. household size), functional (e.g. social support) and quality (e.g. feeling close) social connection factors in relation to adolescent internalising and externalising symptoms, comparing two countries Brazil and the United Kingdom (UK). Methods: We pooled data from the UK Millennium Cohort Study (MCS) and the Brazilian High Risk Cohort Study (BHRCS). We included 12 social connection variables, identified through retrospective harmonisation and lived experience expert involvement. We tested measurement invariance and conducted multiple regressions to analyse associations between the social connection factors (age 14) and later internalising and externalising difficulties (age 17.5) in both cohorts. We investigated country-level interactions and used weights to account for attrition, survey design, population representativeness and sample size. Results: We found pooled main associations with later internalising symptoms for ‘living with half-siblings’ (p <.001), ‘moving address’ (p =.001), ‘mother marital status’ (p <.001–.003), ‘bullying’ (p =.001), ‘being bullied’ (p <.001) and ‘difficulties keeping friends’ (p <.001). For externalising, we found main associations with ‘household size’ (p =.041), ‘moving address’ (p =.041), ‘mother's marital status’ (p =.001–.013), ‘bullying others’ (p <.001) and ‘being bullied’ (p <.001). Country-level interactions suggested higher internalising symptoms were associated with ‘household size’ (p =.001) in Brazil and ‘being bullied’ (p <.001) in MCS. Additionally, ‘half-siblings in household’ (p =.003), ‘poor mother–child relationship’ (p =.018), ‘single mother’ (p =.035), ‘bullying’ (p <.001) and ‘being bullied’ (p <.001) were more strongly linked to externalising difficulties in MCS. Conclusions: Social connection factors, mostly structural, contributed to adolescent internalising and externalising difficulties in both countries. Factors relating to bullying and family composition seem to play a stronger role in each country. Cultural and socioeconomic factors might explain these differences. Future research should investigate cross-regional differences to meaningfully inform global mental health efforts.