Cousin

A cousin is a relative that is the child of a parent's sibling; this is more specifically referred to as a first cousin.

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More generally, in the kinship system used in the English-speaking world, a cousin is a type of relationship in which relatives are two or more generations away from their most recent common ancestor. For this definition degrees and removals are used to further specify the relationship.

Degree measures how distant the relationship is from the most recent common ancestor(s). If the cousins do not come from the same generation, removal is specified, as removal measures the difference in generations between the two cousins. When the removal is not specified, no removal is assumed.^[1][2]

Various governmental entities have established systems for legal use that can precisely specify kinship with common ancestors any number of generations in the past; for example, in medicine and in law, a first cousin is a type of third-degree relative.^{[citation needed]}

People are related with a type of cousin relationship if they share a common ancestor, and are separated from their most recent common ancestor by two or more generations. This means neither person is an ancestor of the other, they do not share a parent (are not siblings), and neither is a sibling of the other's parent (are not the other's aunt/uncle nor niece/nephew).^[2] In the English system the cousin relationship is further detailed by the concepts of degree and removal.

The degree is the number of generations subsequent to the common ancestor before a parent of one of the cousins is found. This means the degree is the separation of the cousin from the common ancestor less one. Also, if the cousins are not separated from the common ancestor by the same number of generations, the cousin with the smallest separation is used to determine the degree.^[1] The removal is the difference between the number of generations from each cousin to the common ancestor.^[1] Two people can be removed but be around the same age due to differences in birth dates of parents, children, and other relevant ancestors.^[1][3][4]

• The terms full cousin^[5] and cousin-german are used to specify a first cousin with no removals.^[6]
• The terms cousin-uncle/aunt and cousin-niece/nephew are sometimes used to describe the direction of the removal of the relationship,^[7] especially in Mennonite,^[8] Indian, and Pakistani ^{[citation needed]} families. These terms relate to a first cousin once removed, uncle/aunt referring to an older generation and niece/nephew for younger ones. For additional removals grand/great are applied to these relationships.^[8] For example, a cousin-granduncle is a male first cousin twice removed that comes from an older generation, and a cousin-grandniece is a female first cousin twice removed who comes from a younger generation.
• The term grandcousin is sometimes used for the grandchild of a first cousin, or the first cousin of a grandparent: a first cousin twice removed.
• The term kissing cousin is sometimes used for a distant relative who is familiar enough to be greeted with a kiss.

Consanguinity is a measure of how closely individuals are related to each other. It is measured by the coefficient of relationship. Below, when discussing the coefficient of relationship, we assume the subject and the relative are related only through the kinship term. A coefficient of one represents the relationship one has with oneself. Consanguinity decreases by half for every generation of separation from the most recent common ancestor, as there are two parents for each child. When there is more than one common ancestor, the consanguinity between each ancestor is added together to get the final result.^[16]

Between first cousins, there are two shared ancestors each with four generations of separation, up and down the family tree: ${\displaystyle \left({\tfrac {1}{2}}\right)^{4}+\left({\tfrac {1}{2}}\right)^{4}}$; their consanguinity is one-eighth. For each additional removal of the cousin relationship, consanguinity is reduced by half, as the generations of separation increase by one. For each additional degree of the cousin relationship, consanguinity is reduced by a quarter as the generations of separation increase by one on both sides.^[16]

Half cousins have half the consanguinity of ordinary cousins as they have half the common ancestors (i.e. one vs two). Double cousins have twice the consanguinity of ordinary cousins as they have twice the number of common ancestors (i.e. four vs two). Double first cousins share the same consanguinity as half-siblings. Likewise, double half cousins share the same consanguinity as first cousins as they both have two common ancestors. If there are half-siblings on one side and full siblings on the other, they would have three-halves the consanguinity of ordinary first cousins.^[16]

In a scenario where two monozygotic (identical) twins have children with another pair of monozygotic twins, the resulting double cousins would test as genetically similar as siblings.

• Collateral descendant
• Consanguinity
• Cousin marriage
• Family
• Sibling
• Second-degree relative