Loneliness and desire for thinness explain sex differences in adolescent internalizing problems

Girls are at higher risk of internalizing problems than boys. This disparity has widened in recent decades, but the underlying mechanisms remain understudied. Here we analysed data from 3,171 adolescents (46.9% girls) within the Tokyo Teen Cohort, assessed at ages 12, 14 and 16 years. At age 16, girls showed higher prevalence of internalizing problems than boys: depressive symptoms (15.9% versus 8.2%) and suicidal ideation (29.8% versus 18.1%). Coproduction with young people suggested loneliness and desire for thinness as candidate pathways. Using the mediational g-formula, we estimated that the effect through these factors at age 14 relative to the total effect of female sex was 34.7% for depressive symptoms and 46.3% for suicidal ideation at age 16. These findings suggest that social intermediates may contribute to the higher prevalence of internalizing problems among girls in this cohort, highlighting potential targets for interventions and policies informed by young people.