Epigenetic rule

Examples of incest in mythology are rampant. Zeus and Hera are brother and sister as well as husband and wife. They were the children of Cronus and Rhea who were also brother and sister as well as husband and wife.

Finally, there is also the much rarer phenomenon of consensual incestuous relations between adults, such as between an adult brother and sister. This is illegal in most places, but these laws are sometimes questioned on the grounds that such relations do not harm other people (provided the couple have no children) and so should not be criminalized. Artificial insemination and distant adoption have compounded these problems. There are known cases of people having romances, or even marrying, only to later find out they are closely related.

Proposals have been made from time to time to repeal these laws--for example, the proposal by the Australian Model Criminal Code Officer's Committee discussion paper "Sexual Offences against the Person" released in November 1996. (This particular proposal was later withdrawn by the committee, in spite of their own feelings on the issue, due to a large public outcry. Defenders of the proposal argue that the outcry was mostly based on the misunderstanding that the committee was intending to legalize sexual relations between parents and their minor children, which it did not.)

Adult incest has been notable in royal dynasties, probably in order to help concentrate wealth and political influence within the family (historical evidence suggests that this practice actually weakened the genetic makeup of elite society family lines, resulting in abnormally high occurrences of rare genetic defects and diseases). Although the marriage unions were often not consensual, with young adults or children forced to marry close relatives, this does not imply the sex was non-consensual. Best known for this practice, which included brother-sister marriages, are the dynasties of Ancient Egypt.

The Tanach (Old Testament) contains prohibitions (primarily in Leviticus) against sexual relations between various pairs of family members. Father and daughter, mother and son, et al., are forbidden on pain of death to engage in sexual relations. An interesting aspect of the Tanach's prohibition of incest is that, according to the interpretation given it by some anthropologists, it prohibits sexual relations between aunts and nephews but not between uncles and nieces.