The primary goal of the present study is to examine the genetic and environmental determinants of aspects of human sexuality, including sexual behaviors which increase the risk of HIV infection ("risky" sexual behaviors). The proposed vehicle for studying such determinants is a twin study, in which the similarity of monozygotic (MZ) twins who have been reared together is compared to that of dizygotic (DZ) twins also reared together. Twins will be recruited from the cohort of the Australian Twin Registry aged 18-25. This cohort contains approximately 4,700 twin pairs, and will yield by far the largest twin sample ever used in the study of sexuality. cooperating twins will be sent a questionnaire consisting of previously- validated items and scales measuring the aforementioned aspects of sexual behavior. Completed questionnaire will provide data for analyses. Current biometrical techniques will be applied to the data. Such techniques should provide, at a minimum, estimates of the importance of genes, shared environment, and non-shared environment for relevant traits, attitudes, and behaviors. Additionally, the determinants of the covariation between traits or behaviors of interest -- particularly, the covariation between sexual orientation and childhood gender nonconformity and the covariation between sexual orientation and childhood gender nonconformity and the covariation among high-risk sexual behaviors -- will be explored. This will help to determine whether such covariation is due to the influence of genes or environmental factors influencing both traits. The primary benefit of this study will be its contribution to theories concerning human sexuality. As the first large scale study to explore genetic and environmental determinants of sexual behavior, this study will have a strong impact on research in the area of human sexuality. Additionally, since the study should provide information regarding the determinants of "risky" sexual behavior, it may also have implications for the modification of such behaviors.