Non-heterosexual (bisexual, lesbian, gay, and asexual) adolescents are at greater risk of common mental health disorders compared to heterosexual adolescents. The dominant theoretical framework used to explain the elevated rates of mental disorders among non-heterosexuals is minority stress theory. However, this model cannot fully explain sexual orientation disparities in mental health outcomes. Thus, other mechanisms or explanations, may help explain the sexual orientation disparities in mental health outcomes should be further explored. In addition, many adult mental health problems begin in childhood or adolescence. Non-heterosexual adolescents may be at greater risk of poorer mental health even in early childhood compared with heterosexual adolescents, which cannot be explained by the minority stress theory since when sexual identity development is nascent. However, the underlying mechanisms remained under-explored. Thus, this project aims to further test the psychosocial and biological influences on the sexual orientation disparities in mental health outcomes, especially in childhood, early adolescence, and the developmental trajectories from childhood to adolescence.