Maartje Boer

SMU AND WELLBEING ACROSS COUNTRIES 117 4 < 0.001; Figure 4.2B). This cross-level interaction explained 55.6% of the country- variance in this association. The country-level prevalence of problematic SMU did not explain any country-variance in this association. In all countries, intense users reported higher levels of friend support than non-intense users ( B = 0.327, p < 0.001; 95% PI = 0.115 to 0.545). The higher the country-level prevalence of intense SMU, the stronger this association was ( B = 0.786, p < 0.001; Figure 4.2B). This cross-level interaction explained 41.7% of the country-variance in this association. Results also suggested that the relationship between intense SMU and friend support was amplified by country-level prevalence of problematic SMU ( B = 1.107, p = 0.036; not shown in Figure). However, the explanatory power of this cross-level interaction was relatively weak, because it explained only 8.3% of the country-variance in this relationship, and it only (marginally) improved AIC, but not BIC (Table 4.2, M6 e relative toM6 d ). Problematic Social Media Use and Wellbeing Figure 4.3 shows the associations between problematic SMUand all wellbeing outcomes according to the models with the best model fits. Mental Wellbeing Figure 4.3A shows that, consistent across countries, problematic users reported lower life satisfaction ( B = -0.823, p < 0.001; 95% PI = -1.179 to -0.467) and more frequent psychological complaints ( B = 0.619, p < 0.001; 95% PI = 0.396 to 0.842) than non-problematic users, although the strength of these associations varied across countries. This country-variance was not related to the country- level prevalence of intense and problematic SMU, as adding these cross-level interactions did not improve model fit (Table 4.2, M1 f,g and M2 f,g ). School Wellbeing Across all countries, problematic users reported lower school satisfaction ( B = -0.316, p < 0.001; 95% PI = -0.480 to -0.152) and higher school pressure ( B = 0.292, p < 0.001; 95% PI = 0.077 to 0.507). The observed country-variances in the strength of these associations were not explained by country-level prevalence of intense and problematic SMU, because adding these cross- level interactions did not improve model fit (Table 4.2, M3 f,g and M4 f,g ).