Cousin

The word cousin refers to a relationship between people who share a certain degree of common ancestry. The word has also been used to refer to common membership in a specific class of people, but that usage is much less common.

Two people are cousins if the nearest common ancestor is a grandparent or more distant ancestor for both people. Where the nearest common ancestor is the parent of one person, and the grandparent or more distant ancestor of the other, the first person is the aunt or uncle of the second, while the second is the niece or nephew of the first.

In informal usage, within an extended family, cousins are those non-sibling relatives who are of approximately similar age, while aunts and uncles are the female and male non-ancestral relatives who are of approximately the age of the parents of the person.

Degrees of cousinship

The relationship most commonly referred to as "cousin" is that of first cousin, where the two people share grandparents. Second cousins are people who share great-grandparents, but no closer relations. As the degree of closest common ancestry increases, the degree of cousinship increases as well.

In Anglo-american usage, when the relationship to the nearest common ancestor is not symmetric, the relationship is described as being "removed". For example, if the nearest common ancestor is the grandparent of one person and the great-grandparent of another, the two are first cousins once removed. The degree of removal is the number of generations difference between the two, while the degree of cousinship is the same as the degree of cousinship at the generation of the person in the earlier generation. Relationships of "Nth cousin P times removed" are symmetric, unlike relationships between aunts and uncles to nephews and nieces.

The following table shows the names of the relationships between descendents of a common ancestor. Find the relationship of person A to the common ancestor along the top of the chart, and the relationship of person B to the common ancestor along the left side of the chart. Go down from the top row in person A's column to the row on which person B is. A is the relation described to B.

Common Ancestor	Child	Grandchild	Great-grandchild	Great-great-grandchild	Great-great-great-grandchild
Child	Sibling (sister or brother)	Niece or nephew	Great-niece/nephew	Great-great-niece/nephew	Great-great-great- niece/nephew
Grandchild	Aunt or uncle	First cousin	First cousin once removed	First cousin twice removed	First cousin three times removed
Great-grandchild	Great aunt or great uncle	First cousin once removed	Second cousin	Second cousin once removed	Second cousin twice removed
Great-great-grandchild	Great-great-aunt/uncle	First cousin twice removed	Second cousin once removed	Third Cousin	Third cousin once removed
Great-great-great-grandchild	Great-great-great- aunt/uncle	First cousin three times removed	Second cousin twice removed	Third cousin once removed	Fourth cousin

Cousin marriage

Marriage between close cousins is strongly taboo and/or illegal in many societies, while it is tolerated or encouraged in others. Most Christian societies have the taboo, and many make marriage between first cousins unlawful. There are two justifications commonly advanced for the persistence of the taboo: genetic diseases and social cohesion. The first is based on the observation that children of siblings or cousins are more likely to express genetic diseases than children of unrelated or distantly related parents; this is due to the increased likelihood that closely-related parents will both be carriers of a genetic disease which is present in their lineage, while unrelated persons are far less likely to both be carriers of the same genetic diseases. The second is based on the observation that where most marriages are between related adults, the society tends to fragment into "clans" or "tribes", with little cohesion outside of the clan or tribe even in the face of external threat; this is partly because family wealth does not disperse, and family loyalties remain undivided.