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In kinship terminology, a cousin is a relative with whom a person shares one or more common ancestors. The term is rarely used when referring to a relative in an immediate family in which there is a more specific term (e.g., mother, father, sister, brother, etc.). The term "blood relative" can be used synonymously and establishes the existence of a genetic link. Systems of "degrees" and "removals" are used in the English speaking world to describe the relationship between two cousins and the ancestor they have in common. Various governmental entities have established systems for legal use that can handle kinships with common ancestors existing any number of generations in the past, though common usage often eliminates the degrees and removals and refers to people with common ancestry as simply "cousins" or "distant cousin".
By extension, the term "cousin" can also be used when referring to the genetic relationships between humans and any other form of life, as per the theory of evolution of all life on Earth descending from one common ancestor. However the term in this sense is most commonly restricted to the fields of study surrounding ecological genetics.
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Although precise legal nomenclature exists, in colloquial speech, "cousin" is used unmodified. Thus a "cousin" may be a first cousin, child of a first cousin, second cousin, cousin's cousin, etc. Modifier terms such as "half-cousin" or "step-cousin" are rarely used in everyday speech. The term "cousin" is also sometimes used for people someone is close to, even if they are not actually blood relatives.
"Second cousin" also often refers to a person's parent's first cousin's child, or someone with whom you share great-grandparents but not grandparents.
A cousin chart, or table of consanguinity, is helpful in identifying the degree of cousin relationship between two people using their most recent common ancestor as the reference point. Cousinship between two people can be specifically described in degrees and removals by determining how close, generationally, the common ancestor is to each person.
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 removal. 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 apart from each other (one "remove"). So, the difference between the degrees is the "removed" part of the equation in this example.
There is a mathematical way to identify the degree of cousinship shared by two individuals. Each "great" or "grand" in the description of one individual's relationship to the common ancestor has a numerical value of 1. For example, if person one's great-great-great-grandfather is person two's grandfather, then person one's "number" is 4 (great + great + great + grand = 4) and person two's "number" is 1 (grand = 1). The smaller of the two numbers is the degree of cousinship. The two people in this example are first cousins. The difference between the two people's "numbers" is the degree of removal. In this case, the two people are thrice (4 − 1 = 3) removed, making them first cousins thrice removed.
Example 2: If someone's great-great-great-grandparent (great + great + great + grand = 4) is another person's great-great-great-grandparent (great + great + great + grand = 4), then the two people are 4th cousins. There is no degree of removal, because they are on the same generational level (4 − 4 = 0).
Example 3: If one person's great-grandparent (great + grand = 2) is a second person's great-great-great-great-great-grandparent (great + great + great + great + great + grand = 6), then the two are second cousins four times removed. The first person's "number" (2) is the lower, making them second cousins. The difference between the two numbers is 4 (6 − 2 = 4), which is the degree of removal (generational difference).
| If one person's → | Grandparent | Great-grandparent | Great-great-grandparent | Great-great-great-grandparent | Great-great-great-great-grandparent | Great-great-great-great-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-great-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-great-great-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-great-great-great-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 first cousins" arise when two siblings of one family reproduce with two siblings of another family. The resulting children are related to each other through both parents' families. Double first cousins share both sets of grandparents in common and have double the degree of consanguinity of ordinary first cousins.
When "identical" twins reproduce with a pair of ordinary siblings, the resulting children are more related than ordinary half-siblings but less related than full siblings (they are genetically equivalent to 3/4 siblings) although they are legally double first cousins.
When identical twins reproduce with another set of identical twins (sometimes called "quaternary marriage"), the resulting children are likewise genetically indistinguishable from full siblings, although they are genealogically double first cousins.
Marriage between double cousins is specifically prohibited in the U.S. State of North Carolina (N.C. Gen. Stat. § 51-3). It is permitted in the other 25 states that permit marriage between first cousins.[1]
When identical twins reproduce with different partners, the resulting children are first cousins but are genetically indistinguishable from half-siblings (because sharing a genetically identical parent is indistinguishable from sharing the same parent). When identical twins reproduce with the same person, the resulting children are likewise genetically indistinguishable from full siblings despite being legally half-siblings. 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 reproduce with the same person, the resulting children are likewise more related than half-siblings but less related than full siblings. Both of these scenarios produce 3/4 siblings. Similar situations arise when two half-siblings marry the same person and when identical twins reproduce with two half-siblings. Children of double first cousins are double second cousins to each other.
Chart relationships as sentences:
Half-siblings share only one parent. Half-cousins share just one grandparent (whereas full first cousins share two grandparents with each other). Half-cousins are the children of two half-siblings (and their different partners). For example, if one of John's parents and one of Mary's parents are half-siblings, then John and Mary are half-first cousins. The half-sibling of each of their respective parents would be their half-aunt or half-uncle but these terms, although technically specific, are rarely used colloquially. While it would not be unusual to hear of another's half-brother, or half-sister, so described, in common usage one would rarely hear of another's half-cousins or half-aunt, so described, and instead hear them described simply as the other's cousin or aunt. Also, children of half-first cousins are half-second cousins to each other and so on because they would share only one common great-grandparent out of eight instead of two, and so on.
Step-cousins are either stepchildren of an individual's aunt or uncle, children of one's step-aunt or uncle, or half-siblings of one's cousin. No blood relationship exists between step-cousins, although there does not need to be a blood relationship between cousins, as in the case of one or both of the siblings being adopted. Step first cousins once removed would be the stepchildren of one's first cousins. A stepchild of one's first cousin would be a step second cousin to their children (they would share a great-grand parent/step great-grand parent).
A cousin-in-law is the spouse of an individual's cousin, similarly the cousin of an individual's spouse, e.g. Owen Henderson will become a cousin-in-law with Nigel Gumbleton when he marries Nigel's cousin, Cerys Baldwin.
A person's cousins may be on either their mother's side or their father's side. If one person's mother's sibling is another's parent, then the second person is the first person's cousin on their mother's side. Similarly, if one person's father's sibling is another's parent, then the second person is the first person's cousin on their father's side. In the Chinese kinship system and some other kinship systems, the terms for cousins related through only paternal relationships are different.
A parallel cousin (or ortho-cousin) is a cousin from a parent's same-sex sibling, while a cross cousin is from a parent's opposite-sexed sibling. So a parallel cousin is the child of the father's brother (paternal uncle's child) or the mother's sister (maternal aunt's child), while a cross cousin is the child of the mother's brother (maternal uncle's child) or of the father's sister (paternal aunt's child). In some cultures, parallel cousins are considered equivalent to siblings, and cousin marriage may be permitted between cross cousins, but not parallel cousins.
The "family relationship" between two individuals a and b, where Ga and Gb respectively are the number of generations between each individual and their nearest common ancestor, can be calculated by the following:
Note that if one of the individuals is the ancestor of the other, one of Ga and Gb is zero and so x is zero.
If 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 consanguinity is expressed as the ordered pair of natural numbers (x, y) as defined above. In that case, 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.
Another visual chart used in determining the legal relationship between two people who share a common ancestor is based upon a diamond shape, 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.
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