Incest

Incest, sexual intercourse between closely related individuals, has been a taboo in most societies, primitive and modern. The concept of what constitutes close relatives, however, is not uniform among societies. At one extreme lie the Trobriand Islanders, who prohibit only a relationship between mother and son, allowing all others—including between father and daughter. At the other extreme are perhaps the Chinese, who prohibit marriages between people sharing the same surname, since they are considered related. Variations also occur in regions within the same country. While as many as 50 percent of marriages in southern India are between first cousins, the same is considered abnormal in northern India. Finally, levirate marriages, between a widow and her husband's brother, are not considered incestuous in most societies and are even encouraged in some.

Rules prohibiting incestuous unions, especially sexual intercourse, do not necessarily coincide with nonrecognition of marriage. Thus, while marriage between two closely related persons is void in India, the same persons can engage in sexual union with impunity. Thus, incest is not a crime in India. Other anomalies exist elsewhere. English law prohibits intercourse between grandfather and granddaughter, but not between grandmother and grandson. The Tanach, the Jewish Bible, contains prohibitions (primarily in Leviticus) against sexual relations between various pairs of family members, such as father and daughter, mother and son, and aunts and nephews, but not between uncles and nieces.

In ancient Babylon, incest between father and daughter was punishable by exile; incest between mother and son was punishable by burning of both persons. In some societies, such as ancient Egypt, incest was the norm. The famous Egyptian Queen Cleopatra (69–30 BCE) married her younger brother, Ptolemy XIII (63–47 BCE). In fact, some historians claim (while others reject the idea) that the cause of the downfall of the Egyptian empire was the growing genetic defects brought about by rampant incestuous marriages in that society.