RESUMEN
Half of all mental health disorders appear during adolescence, although it is still far from clear how they relate to gender (not sex) criteria. This study aims both to analyse the relationship between gender and adolescent mental health and to propose an index: the Gender Adherence Index (GAI). We used cross-sectional, secondary data from 3888 adolescents (aged 13-19) from the FRESC Health Survey on Adolescence in Barcelona. We analysed the interaction among sex, age and socio-economic status with several mental health indices. Additionally, we computed a Gender Adherence Index (GAI) to transcend the information-poor binary sex label, and thus assess to what extent mental health can be predicted by the gender expression of adolescents irrespective of their biological sex. We found that older age and lower economic status have a greater impact on the emotional distress of girls, who reported lower self-perceived mental health than boys. Nevertheless, girls obtained higher scores regarding their prosocial behaviour, which is protective against mental health problems. The GAI was retained in all statistical models stressing it as a relevant metric to explain the variability of adolescent emotional distress. Young people who showed adherence to normative femininity in their lifestyles showed higher prosocial behaviour but did not tend to present more emotional distress. Despite its limitations, this is a novel attempt to explore the relationship between gender expression and mental health. Better defined indices of gender adherence could help us to improve our predictive capacity of mental health disorders during adolescence.