Despite differences in research traditions, the disciplines of genetics, epidemiology, and demography are becoming increasingly integrated in health-related research. The enormous development within genetic technology, with the possibility of genotyping thousands of variants from small samples of biological material obtained by non-invasive methods, now makes it feasible to include genetic information in epidemiologic and demographic studies. Simultaneously, new insight can be obtained from hybrids of methods and data from the three disciplines. This paper illustrates how a genetic observation combined with demographic insight and a modified genetic-epidemiologic design (a twin study) provides evidence that part of the sex difference in survival can be attributed to the fact that females have two X chromosomes and males have only one, a result that is of potential interest for genetics, epidemiology, and demography.