Prohibited degree of kinship
In law, a prohibited degree of kinship refers to a degree of consanguinity (blood relatedness) and sometimes affinity (relation by marriage or sexual relationship) between persons that results in certain actions between them being 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 sexual relations between parent and child or two full-blooded siblings is a cultural universal. Taboos against sexual relations between individuals of other close degrees of relationship vary, but stigmatization of unions with full siblings and with direct descendants are widespread.
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. Conservative Lutherans also prohibit marriage within close degrees of consanguinity and affinity, even if not specifically outlawed by the state. 
Medieval canon law
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).
In Australia, the Marriage Act 1961 prohibits a marriage to a direct ancestor or descendant or sibling (whether full sibling or half sibling), including those arising from a legal adoption. Such marriages are void.
England and Wales, and the World-wide Anglican Communion
The 1662 Book of Common Prayer of the Church of England, long used in various forms through a broad swathe of Anglicanism, included a Table of Kindred and Affinity listing the prohibited degrees of kinship within which one could not marry describing in detail the cases in which marriage was forbidden due to consanguinity or marital affinity. The list was enacted by the Marriage Act 1949 which with significant changes continues to apply in England and Wales. The list was cut back by the Marriage (Prohibited Degrees of Relationship) Act 1986, by deleting from the list prohibitions based on affinity relationships, and added to in other respects. Marriages that continue to be prohibited in England and Wales by the 1949 Act are as follows:
The Children Act 1975 added the following prohibitions:
|Prohibited to men||Prohibited to women|
|Adoptive mother or former adoptive mother||Adoptive father or former adoptive father|
|Adoptive daughter or former adoptive daughter||Adoptive son or former adoptive son|
The Marriage Act 1949 also prohibited marriage to the following affinity relations, but these were repealed by the Marriage (Prohibited Degrees of Relationship) Act 1986:
|Wife's mother||Husband's father|
|Wife's daughter||Husband's son|
|Father's wife||Mother's husband|
|Son's wife||Daughter's husband|
|Father's father's wife||Father's mother's husband|
|Mother's father's wife||Mother's mother's husband|
|Wife's father's mother||Husband's father's father|
|Wife's mother's mother||Husband's mother's father|
|Wife's son's daughter||Husband's son's son|
|Wife's daughter's daughter||Husband's daughter's son|
|Son's son's wife||Son's daughter's husband|
|Daughter's son's wife||Daughter's daughter's husband|
The Marriage (Prohibited Degrees of Relationship) Act 1986 prohibits a marriage to the following, until both parties are aged 21 or over, and provided that the younger party has not at any time before attaining the age of 18 been a child of the family in relation to the other party:
|Daughter of former wife||Son of former husband|
|Former wife of father||Former husband of mother|
|Former wife of father’s father||Former husband of father’s mother|
|Former wife of mother’s father||Former husband of mother’s mother|
|Daughter of son of former wife||Son of son of former husband|
|Daughter of daughter of former wife||Son of daughter of former husband|
The Marriage (Prohibited Degrees of Relationship) Act 1986 also prohibits a marriage to the following:
|Mother of former wife, until the death of both the former wife and the father of the former wife||Father of former husband, until after the death of both the former husband and the mother of the former husband|
|Former wife of son, until after the death of both his son and the mother of his son||Former husband of daughter, until after the death of both her daughter and the father of her daughter|
The Marriage Act 1949 (Remedial) Order 2007 accepted the ruling of the European Court of Human Rights and removed the ban on marriage with a former mother-in-law/daughter-in-law.
As of February 2010[update], 30 U.S. states prohibit most or all marriage between first cousins, and a bill is pending in Maryland that would prohibit most first cousins from marrying. Six states prohibit marriages between first cousins once removed. Some states that prohibit cousin marriage recognize cousin marriages performed in other states.
Virginia rulings in Jaques v. Commonwealth, 51 Va. (10 Gratt.) 690 (1853), stated the long-standing, common-law rule disqualifying a venireman (juror) who is related, within the ninth degree of consanguinity or affinity, to a party to a suit.
- Christian Cyclopedia article on Impediments to Marriage.
- de Colquhoun, Patrick MacChombaich, A summary of the Roman civil law (William Benning and Co., Cambridge, 1849), p. 513
- Bouchard, Constance B., 'Consanguinity and Noble Marriages in the Tenth and Eleventh Centuries', Speculum, Vol. 56, No. 2 (Apr., 1981), p. 269
- Bouchard, Constance B., 'Consanguinity and Noble Marriages in the Tenth and Eleventh Centuries', Speculum, Vol. 56, No. 2 (Apr., 1981), pp. 269-70
- Bouchard, Constance B., 'Consanguinity and Noble Marriages in the Tenth and Eleventh Centuries', Speculum, Vol. 56, No. 2 (Apr., 1981), p. 270
- The History of Medieval Canon Law in the Classical Period, 1140-1234, Ed. Wilfried Hartmann, Kenneth Pennington (The Catholic University of America Press, 2008), p. 377
- "Marriage Act 1961, s 23B".
- Table of Kindred and Affinity
- "Marriage Act 1949 (c. 76)". The UK Statute Law database. Retrieved 30 Aug 2010.
- "Marriage (Prohibited Degrees of Relationship) Act 1986 (c. 16)". The UK Statute Law database. Retrieved 30 Aug 2010.
- "Marriage Act 1949 (Remedial) Order 2007". The UK Statute Law database.
- McGlothlen, John (February 18, 2010). "Maryland lawmaker: Ban first-cousin marriages as unsafe". The Gazette. Archived from the original on 2016-11-29.
- The Love That Dare Not Speak Its Surname
- Wolfson, Evan (2004). Why marriage matters: America, equality, and gay people's right to marry. Simon & Schuster. p. 151. ISBN 978-0-7432-6458-7.
- Patterson Bank v. Gunter, 588 S.E.2d 270 (2003) 263 Ga. App. 424
- Gray v. Com., 311 SE 2d 409 - Va: Supreme Court 1984
- Criminal Law (Sexual Offences and Related Matters) Amendment Act, 2007, s. 12.