A survey of all the papers relating to life span in Drosophila melanogaster published in Experimental Gerontology from its origin in 1964 to 1981 shows that, contrary to a common belief, the mean life span of females exceeds that of males in only approximately 50% of the cases. It is shown that mean life span, as it is measured in most experiments, is a poor estimate of the potential life span of a Drosophila strain. However, the analysis of four extensive studies of Drosophila melanogaster life span strongly suggests that the potential (or maximal) life span is consistently higher for females than it is for males. That analysis also shows that, vis-à-vis the controlled or uncontrolled variations of the environment, the males have a broader norm of reaction or, in other words, a smaller homeostasis than the females. A model, mainly based on these two results, allows us to explain how the mean life span of males is so often higher than that of females.