Young pregnancy and the behaviors that lead to it—penile-vaginal sexual intercourse and contraceptive non-use—have important long-term consequences for health and well-being. Religiosity plays an important role in these behaviors; more than half of U.S. states require that sex education curricula include an emphasis on abstinence, a faith-based approach to avoiding undesired pregnancy. We propose to build on existing research using cross-sectional and large-interval longitudinal datasets, which have dramatically advanced our understanding of how religious and non-religious youth differ in terms of overall pregnancy rates and related behaviors, but do not include detailed measures of the dynamic behaviors that lead to pregnancy. We will use the only available population-based dataset with repeated weekly measures of penile-vaginal sex and contraceptive behaviors and the intimate relationships in which they occur, combined with detailed questions about religious affiliation, beliefs, and behaviors. The questions about religion permit us to construct three distinct measures of religiosity across three domains: religious conservatism (affiliation with a conservative denomination, belief that the bible is the inerrant word of God), external religiosity (service attendance), and internal religiosity (praying, religious salience). We propose to estimate differences across these domains of religiosity in terms of an integrated set of sexual and contraceptive behaviors throughout the study period—time spent in an intimate partnership, time to first penile-vaginal intercourse within each intimate relationship, frequency of penile-vaginal intercourse after a relationship became sexual, whether any contraceptive method was used during each week that included penile-vaginal intercourse, whether a hormonal (vs. coital) method was used during each week of contraceptive use, whether a condom was used consistently (during each act of penile-vaginal intercourse) during weeks of condom use, and whether the male partner consistently withdrew the penis before ejaculation during each week withdrawal was used. We will also analyze pregnancy rates and undesired pregnancy rates and estimate the extent to which differences by religiosity are due to differences in penile-vaginal sexual behavior, differences in contraceptive use, or both. The combined detail of these analyses is unprecedented and will allow us to distinguish among multiple mechanisms through which religiosity influences behavior, such as conservative values about pre-marital sex, fear of pregnancy, lack of planning for penile-vaginal intercourse, beliefs about the beginning of life, social exposure, social interaction, and social control. We propose the ideal collaborative research team for these analyses. Barber was the PI of the original RDSL project, and Pearce designed the religiosity measures in the RDSL baseline survey. Their complementary expertise—Barber’s in intimate relationships, sexual behavi...