Prohibited degree of kinship

The prohibited degree of kinship refers to a degree of consanguinity (relatedness) below which sexual interrelationships are regarded as incestuous. Inbreeding is a taboo across nearly all cultures worldwide, but the line at which a relationship is considered incestuous varies. The prohibited degree of kinship may exist between siblings, half-siblings, first cousins, third cousins, etc.

In Catholicism
In the Roman Catholic Church, unwittingly marrying a closely-consanguinious blood relative is grounds for an annulment, but dispensations were granted, actually almost routinely (the Roman Catholic Church's ban on marriage within the fourth degree of relationship (third cousins) lasted from 1550 to 1917; before that, the prohibition was to marriages between as much as seventh degree of kinship).

The relevant Latin Rite Canon Law in force since 1983 is as follows: [http://www.vatican.va/archive/ENG1104/__PC.HTM Can. 108]
 * 1. Consanguinity is computed through lines and degrees.
 * 2. In the direct line there are as many degrees as there are generations or persons, not counting the common ancestor.
 * 3. In the collateral line there are as many degrees as there are persons in both the lines together, not counting the common ancestor.

Can. 109
 * 1. Affinity arises from a valid marriage, even if not consummated, and exists between a man and the blood relatives of the woman and between the woman and the blood relatives of the man.
 * 2. It is so computed that those who are blood relatives of the man are related in the same line and degree by affinity to the woman, and vice versa.

Can. 110
 * Children who have been adopted according to the norm of civil law are considered the children of the person or persons who have adopted them.

[http://www.vatican.va/archive/ENG1104/__P3Y.HTM Can. 1091]
 * 1. In the direct line of consanguinity marriage is invalid between all ancestors and descendants, both legitimate and natural.
 * 2. In the collateral line marriage is invalid up to and including the fourth degree.
 * 3. The impediment of consanguinity is not multiplied.
 * 4. A marriage is never permitted if doubt exists whether the partners are related by consanguinity in any degree of the direct line or in the second degree of the collateral line.

Can. 1092
 * Affinity in the direct line in any degree invalidates a marriage.

Can. 1093
 * The impediment of public propriety arises from an invalid marriage after the establishment of common life or from notorious or public concubinage. It nullifies marriage in the first degree of the direct line between the man and the blood relatives of the woman, and vice versa.

Can. 1094
 * Those who are related in the direct line or in the second degree of the collateral line by a legal relationship arising from adoption cannot contract marriage together validly.

Canons 1091.2, 1092, 1093, 1094 represent dispensable ecclesiastical law, and as such do not apply to the marriage of two non-Catholics per canon 11.

In the Eastern Church
Until the 20th century the Russian Orthodox Church explicitly prohibited marriage within seven degrees of kinship. Many Old Believer groups maintain the prohibition to this day. Nevertheless, sexual relations between in-laws (in particular, snokhachestvo) were fairly common in Imperial Russia.

In Asian Cultures
Up until recently, in certain Asian Cultures, it was forbidden to marry those with the same surname regardless of relations. These clan marriages were considered incestuous. However, first cousins with different surnames were allowed to marry. For example, one can marry their mother's sibling's child, but could not marry a non-blood related stranger because having the same surname was considered as having the same ancestor. However, these rules were not clearly defined. In Korean culture for example, surnames were designated by region as well as the name itself. So, a "Kim" family originating from a northern region was not considered the same as one that originated from a southern region, making marriage between the two allowable. In Hindu belief, the scripture Manusmriti states one cannot marry one who is less than seven generations away from his/her father's side and five from his/her mother's side.

The Church of England
The Church of England traditionally follows the rules set out in the Book of Common Prayer which includes a "Table of Kindred and Affinity". This states that:


 * A Man may not marry his mother, daughter, adopted daughter, father's mother, mother's mother, son's daughter, daughter's daughter, sister, wife's mother, wife's daughter, father's wife, son's wife, father's father's wife, mother's father's wife, wife's father's mother, wife's mother's mother, wife's daughter's daughter, wife's son's daughter, son's son's wife, daughter's son's wife, father's sister, mother's sister, brother's daughter, sister's daughter.


 * A Woman may not marry with her father, son, adopted son, father's father, mother's father, son's son, daughter's son, brother, husband's father, husband's son, mother's husband, daughter's husband, father's mother's husband, mother's mother's husband, husband's father's father, husband's mother's father, husband's son's son, husband's daughter's son, son's daughter's husband, daughter's daughter's husband, father's brother, mother's brother, brother's son, sister's son.

It further states that the term 'brother' includes a brother of the half-blood, and the term 'sister' includes a sister of the half-blood.