I study sex. Don’t get too excited: I study sex in Drosophila fruit-flies and in bluebirds, and sometimes in mallards and mice-most often in the field and sometimes in the lab. I am interested in the reproductive decisions that males and females make, i.e., when and with whom to mate and reproduce, and what factors influence or constrain these decisions. I also study mathematical models of mating decisions in individuals and in populations. I am testing my hypothesis that freely made mating decisions in both sexes are about having healthy kids. My colleagues and I recently did experiments to test whether preferring one’s mating partner mattered to offspring viability. It does: the offspring of both males and females who were mated with a partner they preferred rather than with one they did not prefer had higher viability in all the species we studied. In another experiment with Drosophila we found that polyandrous females (females who mate with more than one male, in our case a new male every day) have more offspring that survive to adulthood than monogamous females who only mate with one male over their adult lives-even though the polyandrous females don’t lay any more eggs than the monogamous females. This fitness difference was not due to differences in lifespan: the lifespans of polyandrous and monogamous females were the same. If this result holds in species besides fruit flies, the occurrence of polyandry-which has been found in most species so far studied-may reflect natural selection due to the offspring viability benefits of polyandry.