Research has demonstrated that eating disorders (ED) and higher weight have lifetime co-occurrence suggesting that they may be best considered within a common etiological model. Although we know that body dissatisfaction is likely to be a risk factor for both outcomes, other proposed risk and protective factors for each condition have not been adequately explored. The current paper tests a conceptual model that is based on a review of the existing literature from both areas of scholarship. It considers biological, sociocultural, psychological, and behavioral factors that may contribute to both outcomes. The model will be tested in a longitudinal design with an initial sample of 600 emerging adults (aged 18-30) per country in nine different countries (total sample = 5400 participants). Questionnaires will be completed online on two occasions, 12 months apart. The first full phase of the study commenced in July 2018, the same time Body Image was approached to publish this protocol paper (the final revised paper was submitted in September 2019), and data collection will be finalized in December 2019. Multi-group path analysis will identify the biopsychosocial predictors - both cross-sectionally and longitudinally - of both ED and higher weight, and how these vary across countries and gender.