Cousin

Family tree showing the relationship of each person to the orange person. Cousins are colored green. Generations are shown by alternating stripes of gray and white.

In kinship terminology, a cousin is a relative with whom one shares a common ancestor. In modern usage, the term is rarely used when referring to a relative in one's own line of descent, or where there is a more specific term to describe the relationship: e.g., brother, sister, aunt, uncle. The term blood relative can be used synonymously, and underlines the existence of a genetic link. A system of degrees and removes is used to describe the relationship between the two cousins and the ancestor they have in common.

The degree (first, second, third cousin, et cetera) indicates one less than the minimum number of generations between both cousins and the nearest common ancestor. For example, a person with whom one shares a grandparent (but not a parent) is a first cousin; someone with whom one shares a great-grandparent (but not a grandparent) is a second cousin; and someone with whom one shares a great-great-grandparent (but not a great-grandparent) is a third cousin; and so on.

The remove (once removed, twice removed, etc.) indicates the number of generations, if any, separating the two cousins from each other. The child of one's first cousin is one's first cousin once removed because the one generation separation represents one remove. Oneself and the child are still considered first cousins, as one's grandparent (this child's great-grandparent), as the most recent common ancestor, represents one degree. Equally the child of one's great-aunt or uncle (one's parent's cousin) is one's first cousin once removed because their grandparent (one's own great-grandparent) is the most recent common ancestor.

Non-genealogical usage often eliminates the degrees and removes, and refers to people with common ancestors merely as cousins or distant cousins. Alternatively, the terms 'second cousin' and 'first cousin once removed' are often incorrectly used interchangeably.

The system can handle kinships going back any number of generations (subject to the genealogical information being available).

Cousin chart

A cousin chart, or table of consanguinity, is helpful in identifying the degree of cousin relationship between two individuals using their most recent common ancestor as the reference point. Cousinship between two individuals can be specifically described in degrees and removes by determining how close, generationally, the common ancestor is to each individual.

Additional modifying words are used to clarify the exact degree of relatedness between the two people. Ordinal numbers are used to specify the number of generations between individuals and a common ancestor, and further clarification of exact cousinship is made by specifying the difference in generational level between the two cousins, if any, by using degrees of remove. For example, "first cousins once removed" describes two individuals with the common ancestor being the grandparent of one cousin (one degree) and the great-grandparent of the other cousin (two "degrees"). The degree of lowest number is considered the degree. The cousins themselves are one generation different from each other (one remove). So, the difference between the degrees is the "removed" part of the equation.

To make this easy to remember, think about it this way. Assign 1 to the word grand and 1 to each great. Figure two people with a common ancestor by assigning each of you the totals of the grand and greats. Example: If your great-great-great grandfather is another person's grandfather, your number is 4 (great + great + great + grand = 4) and the other person's number is 1 (grand = 1). Which number is smaller? 1. So, you are first cousins. The smallest of the two persons' numbers is the degree. Now, what is the difference between the two numbers? 4 - 1 = 3 So, you are 3 times removed.

Example 2: If your great-great-great grandparent (great + great + great + grand = 4) is another person's great-great-great grandparent (great + great + great + grand = 4), then you are 4th cousins. There is no removed, because you are on the same generational level (4 - 4 = 0).

Example 3: If your great grandparent (great + grand = 2) is the second person's great-great-great-great-great grandparent (great + great + great + great + great + grand = 6), then you are second cousins, four times removed. Why? Because your number is the lowest, being 2 (so, the degree is second cousin), and the difference between your two numbers is 4 (6 - 2 = 4), which is the number of removes (generational difference).

If one person's →		Grandparent	Great-grandparent	Great-great-grandparent	Great-grandparent	Great-grandparent	Great-grandparent
is the other person's ↓	then they are ↘
Grandparent		1st cousins	1st cousins once removed	1st cousins twice removed	1st cousins thrice removed	1st cousins four times removed	1st cousins five times removed
Great-grandparent		1st cousins once removed	2nd cousins	2nd cousins once removed	2nd cousins twice removed	2nd cousins thrice removed	2nd cousins four times removed
Great-great-grandparent		1st cousins twice removed	2nd cousins once removed	3rd cousins	3rd cousins once removed	3rd cousins twice removed	3rd cousins thrice removed
Great-grandparent		1st cousins thrice removed	2nd cousins twice removed	3rd cousins once removed	4th cousins	4th cousins once removed	4th cousins twice removed
Great-grandparent		1st cousins four times removed	2nd cousins thrice removed	3rd cousins twice removed	4th cousins once removed	5th cousins	5th cousins once removed
Great-grandparent		1st cousins five times removed	2nd cousins four times removed	3rd cousins thrice removed	4th cousins twice removed	5th cousins once removed	6th cousins

Double cousins

Generally, one's cousinship to another is determined by a connection through only one parent's biological family. But an individual's cousinship to another individual may be determined by a connection through both of one's parents. These cousins are biologically connected to both the maternal and paternal family trees and that cousinship is termed a double cousin. Another term used to describe this is cousins on both sides.

If a pair of siblings from one family each form a couple with a pair of siblings from another family, then the children of these two couples will be double first cousins to one another. The children of the couples would already automatically be first cousins because they are children of one of their parent's siblings, but in this case the children of their mother's sibling are also the children of their father's sibling, and thus they are double first cousins. Such cousins have double the consanguinity of ordinary cousins and are as related as half-siblings. Instead of the 12.5% consanguinity that simple first cousins share with each other, double first cousins share a 25%consanguinity with each other. Further, when identical twins form a coupling with a corresponding set of identical twins, the children of these two couples, though legally (double) first cousins to one another, would genetically be as closely related to each other as ordinary full siblings.

Other types of cousin

When identical twins reproduce with the same person, the resulting children are likewise genetically indistinguishable from full siblings, although they are legally half-siblings and first cousins. When identical twins reproduce with siblings the resulting children are more related than half-siblings but less related than full siblings. When two siblings who are not identical twins marry the same person, the resulting children are likewise more related than half-siblings but less related than full siblings. The same situation arises when two half-siblings marry the same person. Children of double first cousins are double second cousins to each other.

Chart relationships as sentences:

• If two first cousin men have children with two first cousin women then these children are double second cousins because they share both sets of great-grandparents on both the maternal and the paternal family trees.

• If two female first cousins have children with two male second cousins, these children are maternal second cousins / paternal third cousins.

• If two siblings procreate with two second cousins then the resulting children would be paternal first cousins and maternal third cousins, or vice versa.

• Inbreeding: If a male and a female third cousins have children, then these children would be siblings / double fourth cousins. (See cousin marriage.) This could be construed as incest in some cultures, especially if the third cousins know that they are related. Technically, it is considered inbreeding as geneticists can easily detect a genetic relationship with third cousins.

• If a male and a female second cousins have children with siblings a brother and sister and then these children are first cousins / double third cousins.

Mathematical definitions

The family relationship between two individuals a and b, where G_a and G_b respectively are the number of generations between each individual and their nearest common ancestor, can be calculated by the following:

x = min (G_a, G_b)

y = |G_a − G_b|

• If x = 0 and y = 0 then they are the same person.
• If x = 0 and y = 1 then they are parent and child.
• If x = 0 and y = 2 then they are grandparent and grandchild.
• If x = 0 and y > 2 then they are great ... great-grandparent and great ... great-grandchild, with y − 2 greats.

• If x = 1 and y = 0 then they are siblings (brothers, sisters or brother and sister).
• If x = 1 and y = 1 then they are uncle/aunt and nephew/niece.
• if x = 1 and y = 2 then they are granduncle/grandaunt and grandnephew/grandniece (or great-uncle/great-aunt and great-nephew/great-niece).
• If x = 1 and y > 2 then they are great ... great-granduncle/great-grandaunt and great ... great-grandnephew/great-grandniece, with y − 2 greats (or great- ... great-uncle/great- ... great-aunt and great- ... great-nephew/great- ... great-niece, with y − 1 greats).

• If x > 1 and y = 0 then they are (x − 1)th cousins. First cousins are usually just called cousins when contrast with more distant relations is not called for.
• If x > 1 and y > 0 then they are (x − 1)th cousins y times removed.

If x > 0 and they only share one nearest common ancestor rather than two, then the word "half" is sometimes added at the beginning of the relationship.

Granduncle/grandaunt and grandnephew/grandniece are equivalent to great-uncle/great-aunt and great-nephew/great-niece. Both great-uncle and granduncle refer to an uncle of one's father or mother. Neither form is definitively more correct than the other.

The mathematical definition is more elegant if you always express consanguinity as the ordered pair of natural numbers (x, y) as defined above. In that case, the relationship one has with oneself is (0, 0), the relationship between parent and child is (0, 1), and the relationship between grandparent and grandchild is (0, 2). The relationship between siblings is (1, 0); and between aunt/uncle and nephew/niece is (1, 1). First cousins are (2, 0). The first number expresses how many generations back the two people's most recent common ancestor is, while the second number expresses the generation difference between the two people.

Alternative canon law charts

Canon Law Relationship Chart.

Another visual chart used in determining the legal relationship between two people who share a common ancestor is based upon a diamond shape, and is usually referred to as a canon law relationship chart.

The chart is used by placing the "common progenitor" (the person from whom both people are descended) in the top space in the diamond shaped chart, and then following each line down the outside edge of the chart. Upon reaching the final place along the opposing outside edge for each person, the relationship is then determined by following that line inward to the point where the lines intersect. The information contained in the common "intersection" defines the relationship.

For a simple example, in the illustration to the right, if two siblings use the chart to determine their relationship, their common parents are placed in the topmost position and each child is assigned the space below and along the outside of the chart. Then, following the spaces inward, the two would meet in the "brother (sister)" diamond. If their children want to determine their relationship, they would follow the path established by their parents, but descend an additional step below along the outside of the chart (showing that they are grandchildren of the common progenitor); following their respective lines inward, they would come to rest in the space marked "1st cousin." In cases where one side descends the outside of the diamond further than the other side because of additional generations removed from the common progenitor, following the lines inward shows both the cousin rank (1st cousin, 2nd cousin) plus the number of times (generations) "removed."

In the example provided at the right, generations one (child) through ten (8th great-grandchild) from the common progenitor are provided; however the format of the chart can easily be expanded to accommodate any number of generations needed to resolve the question of relationship.

See Also

• Ancestry
• Ancestor
• Classificatory kinship
• Common descent
• First-cousin marriage
• Line of descent

License

This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Local history".

This article is considered to be one of the best articles in GeneaWiki.