While much research focused on how self-beliefs, such as "I am worthless", alter self-perception(s) and in turn health outcomes, little is known about how our beliefs about the world, such as “the world is safe”, relate to mental (and physical) health. Recently, Clifton et al. (2019) developed a questionnaire identifying core beliefs about the general character of the world – so-called primal world beliefs, or primals for short. Since then several studies have suggested stability of primals and strong associations with mental as well as physical health, often interpreted as causal. Importantly, to our knowledge, primals research to date is entirely based on cross-sectional data and we are the first (and only) who have collected primals information in a genetically informative twin sample, making it possible to address causality and familial confounding for the first time. In this project, we make use of health and registry data as well as twin and genotype data, to investigate the genetic etiology of primal world beliefs and their associations with other traits.