Consanguinity

Consanguinity, from the Latin *cōnsanguinitās* meaning "blood relationship" or "kinship," refers to being related to someone through descent from a common ancestor. This concept has shaped legal, religious, and social norms worldwide, influencing rules on marriage, inheritance, and sexual relations, while also being a common practice in about 20% of the global population.

Many legal systems restrict marriage or sexual relations between close blood relatives, though the exact degree of relation that triggers these prohibitions varies by jurisdiction. Conversely, in some regions, consanguineous marriages are preferred. The degree of kinship also determines inheritance rights under intestate succession laws, which differ from place to place. Attitudes toward cousin marriage range from acceptance or encouragement to outright taboo, sometimes classified as incest.

The degree of consanguinity can be mapped using a consanguinity table, where each row represents a generation (or meiosis), and collateral relatives appear on the same row. The Knot System provides a numerical method for describing consanguinity based on the Ahnentafel numbers of shared ancestors.

**Legal definitions**

**Modern secular law** The degree of kinship between two people can raise several legal issues. Some laws prohibit sexual relations between close relatives, labeling them incestuous. Marriage between closely related people is also often barred, almost universally up to the second degree of consanguinity. Some jurisdictions forbid first-cousin marriage; others do not. Marriage with aunts or uncles (avunculate marriage) is legal in several countries. Consanguinity also affects inheritance, especially in intestate succession, where laws generally favor closer relatives. Some jurisdictions disqualify people from jury service based on consanguinity or affinity with parties in a case. In many countries, nepotism laws restrict public officers from employing or contracting with close relatives.

**Religious and traditional law**

**Judaism** (No specific additional facts provided beyond the general scope.)

**Christianity** Under Roman civil law, which early Catholic canon law followed, marriage was forbidden within four degrees of consanguinity. Around the ninth century, the church expanded the prohibition to seven degrees and changed the calculation method: instead of counting generations up to the common ancestor and back down, it counted only the generations back to the common ancestor. This meant marriage was prohibited to anyone more closely related than seventh cousins, severely limiting marriage options for the nobility, who often had to defy the church or seek spouses elsewhere. In the Roman Catholic Church, unknowingly marrying a close blood relative could lead to a declaration of nullity, but by the eleventh and twelfth centuries, dispensations became more common due to the vast number of people covered by the seven-degree ban. In 1215, the Fourth Lateran Council reduced the prohibited degrees back to four, while keeping the newer calculation method. After that, fourth cousins could generally marry without dispensation, reducing the need for them. In fourteenth-century England, papal dispensations for annulments based on consanguinity (and affinity) were relatively rare. The Catholic Church’s ban on minor-degree marriages faced heavy criticism in eleventh-century Croatia, leading to a schism in the Croatian church.

Among Christian Habesha highlanders in Ethiopia and Eritrea (primarily Orthodox Christian Amhara and Tigray-Tigrinya), tradition requires recounting paternal ancestors at least seven generations back from early childhood. Those sharing a common patrilineal ancestor less than seven generations away are considered "brother and sister" and cannot marry. The rule is less strict on the mother’s side, with a limit of about four generations, but still determined patrilineally. This rule does not apply to Muslims or other ethnic groups.

**Islam** The Quran (4:22–24) lists forbidden marriage partners: mothers, daughters, sisters, father’s sisters, mother’s sisters, brother’s daughters, and sister’s daughters. First cousins are not included in this list. Muhammad himself married his first cousin Zaynab bint Jahsh. Some countries offer financial incentives to discourage consanguineous marriages; for example, the UAE (since 2004) and Qatar (since 2009) require mandatory premarital screening for inherited blood disorders, and couples with positive results forfeit their marriage grant.

**Genetic definitions** Genetically, consanguinity reduces variation due to meiosis because of the smaller number of near ancestors. Since all humans share 99.6% to 99.9% of their genome, consanguinity affects only a tiny portion of the sequence. If two siblings have a child, that child has only two grandparents instead of four, increasing the chance of inheriting two copies of a harmful gene.