 Heinlein often posed situations where the nominal purpose of sexual taboos was irrelevant to a particular situation, due to future advances in technology.For example, in "Time Enough for Love" Heinlein describes a brother and sister (Joe and Llita) who were mirror twins, being complementary diploids with entirely disjoint genomes, and thus not at increased risk for unfavorable gene duplication due to consanguinity.In this instance, Llita and Joe were props used to explore the concept of incest, where the usual objection to incest—heightened risk of genetic defect in their children—was not a consideration.