The Swedish Twin Registry (STR) is the world's largest population-based register of twins for scientific research. It has been an invaluable resource for international biomedical researchers studying genetic causes of human diseases since its inception in the late 1950s. Many biomedical scientists consider twin research the "gold standard" for distinguishing between genetic and environmental factors in causing diseases. Most recently, behavioral scientists have been using the STR to generate data about interactions of genes and environments in relation to the heritability of complex human traits and behaviors. In the past ten years, twin studies have been used to suggest the influence of genetics in a widening sphere of human experiences, including risk-taking behaviors, cooperation, and spirituality. This ethnographic project will be situated in and around the Stockholm-based STR and will focus on the networks of Swedish scientists currently using STR resources to study genetic and environmental influences in human behavior. My research objective is to study how Swedish scientists within three disciplines - behavior genetics, psychology, and economics - actually design, execute, and analyze experiments using Swedish twins to produce broader knowledge about the heritability of behaviors, including risk-taking and decision-making in humans. This project will focus on how ways of drawing boundaries between genetic and environmental factors of human heredity might simultaneously transform both science and society. The STR's location in Sweden offers an opportunity to study the production of genetic knowledge within the specific context of the Swedish welfare state.