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

Background: Social connection plays a key role in young people’s mental health. It is important to know which components are involved but also how their influence might vary across contexts. We investigated the prospective association of multiple social connection factors with subsequent adolescent internalising and externalising symptoms comparing Brazil and the United Kingdom (UK).Methods: We harmonised and pooled data from the UK Millennium Cohort Study (MCS) and the Brazil High Risk Cohort Study (BHRCS). We identified 12 common social connection variables through measurement harmonisation and by involving young people with lived experience from both countries. We tested for measurement invariance and conducted multiple regression models. We analysed associations between the social connection factors (age 14) and later internalising and externalising difficulties (age 17.5) in both cohorts. We tested for country-level interactions and used weights to account for attrition, survey design, population representativeness and sample size. Results: Internalising and externalising measures reached scalar invariance. We found pooled main association for six social connection factors and later internalising problems, and five social connection factors with later externalising symptoms. We found some specific country-level associations. Higher internalising symptoms at follow-up were associated with previous higher ‘number of people in household’ in Brazil and ‘being bullied’ in MCS. Additionally, ‘half siblings in households’, ‘poor mother-child relationship’, ‘being a single mother’ and both practised or suffered bullying were more strongly linked to externalising difficulties in MCS than in BHRCS.Conclusions: We show that social connection factors contribute to adolescent internalising and externalising difficulties in both countries. However, some 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 in more detail to meaningfully inform global mental health efforts.