Prohibited degree of kinship

In law, a prohibited degree of kinship refers to a degree of consanguinity (blood relatedness) between persons that results in certain actions between them becoming illegal. Two major examples of prohibited degrees are found in incest and nepotism. Incest refers to sexual relations and marriage between closely related individuals; nepotism is the preference of blood-relations in the distribution of a rank or office.

An incest taboo against relations between parent and child or two full-blooded siblings is a cultural universal. Taboos against sexual relations between individuals of other degrees of close relationship vary between the world's cultures, but stigmatization of unions with full siblings and with direct descendants are widespread.

The Roman Catholic Church and Eastern Orthodox Church have a long history of various marital prohibitions. These function to limit intermarrying between two closely related relatives. In the Book of Common Prayer of the Church of England published since 1662 there is a Table of Kindred and Affinity prohibiting certain marital relationships between relatives, and these provisions were subsequently enacted in statute law in England among other countries. In South Korea it had historically been forbidden to marry someone with the same surname and clan regardless of the distance of the relation, but this law was ruled unconstitutional in 1999.

Roman civil law prohibited marriages within four degrees of consanguinity. This was calculated by counting up from one prospective partner to the common ancestor, then down to the other prospective partner. The first prohibited degree of consanguinity was a parent-child relationship while a second degree would be a sibling relationship. A third degree would be an uncle/aunt with a niece/nephew while fourth degree was between first cousins. Any prospective marriage partner with a blood relationship outside these prohibited degrees was considered acceptable.Canon law followed civil law until the early ninth century, when the Western Church increased the number of prohibited degrees from four to seven. The method of calculation was also changed to simply count the number of generations back to the common ancestor. This meant that marriage to anyone up to and including a sixth cousin was prohibited. The Fourth Lateran Council of 1215 decreed a change from seven prohibited degrees back to four (but retaining the same method of calculating; counting back to the common ancestor).

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