Prohibited degree of kinship

Roman civil law prohibited marriages within four degrees of consanguinity.[1] This was calculated by counting up from one prospective partner to the common ancestor, then down to the other prospective partner.[2] 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.[2] Any prospective marriage partner with a blood relationship outside these prohibited degrees was considered acceptable.[2]

The Roman Catholic Church and Eastern Orthodox Church have a long history of marital prohibitions, called impediments to marriage, which limit the marriage of two closely related relatives. Initially, canon law followed Roman civil law until the early 9th century, when the Western Church increased the number of prohibited degrees from four to seven.[3] The method of calculation was also changed to simply count the number of generations back to the common ancestor.[4] 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).[5]

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.[6]

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.[7] The list was enacted by the Marriage Act 1949 which with significant changes continues to apply in England and Wales.[8] 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:

The Marriage Act 1949 also prohibited marriage to the following affinity relations, but these were repealed by the Marriage (Prohibited Degrees of Relationship) Act 1986:[9]

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:

The Marriage (Prohibited Degrees of Relationship) Act 1986 also prohibits a marriage to the following:

The Marriage Act 1949 (Remedial) Order 2007[10] 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.

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. [11]

Laws regarding first-cousin marriage in the United States

  First-cousin marriage is legal

  Allowed with requirements or exceptions

  Banned with exceptions¹

  Statute bans first-cousin marriage¹

  Criminal offense¹

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¹Some states recognize marriages performed elsewhere, especially when the spouses were not residents of the state when married.

As of February 2010, 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.[12] Six states prohibit marriages between first cousins once removed.[13] Some states that prohibit cousin marriage recognize cousin marriages performed in other states.[14]