Question: Why is incest a sin (Lev 18:6) when the son's and daughters of Adam and Eve intermarried? Would like other's opinions on this topic. My reasoning, though not very well developed, is that incest was not forbidden because there were no other choices. I seem to recall some time back Helen giving this subject a good response. It had to do with genetic mutations or human genome (why children born from incest often results in birth defects). That was not the case in the dawn of civilization because the genetic code was pure, but became contaminated by mutations over time. Once the population reached a certain point, God prohibited further incest to protect the human genome. Who wants to take a shot at this?