The overall aim of this project is to investigate the contribution of biological and psychosocial factors early in life to sexual orientation later in life. Scholars from across the natural sciences, social sciences, and humanities have argued that differences in human sexuality and gender behaviour are shaped by a rich complexity of biopsychosocial factors including genetics, environment, and culture. However, no studies have robustly tested this “biopsychosocial approach”. For example, previous studies are limited by relying on peoples' reports of their past or on “snap shots” of groups of people at one time point which may produce biases in the data. This study aims to avoid these limitations by exploring how early life factors (including factors such as birth weight, number of siblings, home life, demographic variables) are related to later sexual orientation. We will use longitudinal statistical analysis to quantify how early life factors influence later sexual orientation, as well as changes within-individuals over time. We also want to see if any relationships we find are influenced by the levels of gender behaviour early in childhood, because gender behaviour is strongly associated with sexual orientation. For example, individuals who are more gender nonconforming in their behaviour might have a different pattern of early life factors compared to gender conforming individuals regardless of sexual orientation. We hope our results will provide greater social and cultural understanding of the diversity in human sexuality and gender.