# Cousin

Commonly, "cousin" refers to a "first cousin", people whose most recent common ancestor is a grandparent.[1] A first cousin is a third-degree relative and used to be known as a cousin-german, though this term is rarely used today.[2]

More generally, cousin is a type of familial relationship in which people with a known common ancestor are both two or more generations away from their most recent common ancestor. This distinguishes a cousin from an ancestor, descendant, sibling, aunt, uncle, niece, or nephew.[3]

Systems of "degrees" and "removals" are used in the English-speaking world to describe the exact relationship between two cousins (in the broad sense) and the ancestor they have in common.[4] Various governmental entities have established systems for legal use that can precisely specify kinship with common ancestors any number of generations in the past.[citation needed] Common usage often eliminates the degrees and removals, and refers to people with common ancestry as simply "distant cousins" or "relatives".[5]

## Basic definitions

Family tree. Cousins are colored green. Generations are shown by alternating stripes of gray and white.

People are related with a type of cousin relationship if they share a common ancestor and the most recent common ancestor is two or more generations away from both people. This means neither person is an ancestor of the other, they do not share a parent (siblings), and neither is a sibling of a common ancestor (aunts/uncles and nieces/nephews).[3]

In the English system the cousin relationship is further detailed by degree and removal. For example, the "second cousin once removed" relationship is a "second-degree cousin" with one removal. When the degree is not specified first cousin is assumed. When the removal is not specified no removal is assumed.

The degree of the cousin relationship is one less than the number of generations before a most recent common ancestor is found (the number of generations prior to the parents). If the cousins don't share the same number of generations to the common ancestor, the smaller number of generations to the most recent common ancestor is used to determine the degree of the cousin relationship.[4] For example, if one of the cousins has to go back two generations before finding the most recent common ancestor and the other has to go back two or more they are first cousins. If one had to go back three generations and the other had to go back three or more they would be second cousins.[6][4]

Removal in the cousin relationship is the number of generations the cousins are separated by. When the cousins are separated by a different number of generations from the most recent common ancestor, the cousin relationship is "removed". The difference between the number of generations for each cousin is the removal.[4] Two people can be removed but be around the same age due to differences in birth dates of parents children and other relevant ancestors.[4] [7][6] For example, if the most recent common ancestor is two generations prior for one person and three generations prior for the other the cousins are separated by one generation and therefore once removed. They are also once removed if the most recent common ancestor is three generations prior for one person and four generations prior for the other (one person's great-grandfather is the other person's great-great-grandfather). This can be is illustrated by following these examples in the table below.

This table is helpful in identifying the degree of cousin relationship between two people using their most recent common ancestor as the reference point, and demonstrates an example in a family tree.

Relative's removal (Relationship to common ancestor) 2 (Grandparent) 3 (Great-grandparent) 4 (Great-great-grandparent) Relationship Example from Basic family tree ↘ 1st cousin Joseph & Julie 1st cousin once removed Joseph & Matt 1st cousin twice removed Joseph & Lyla 1st cousin ${\textstyle \left\vert R-1\right\vert }$ times removed 1st cousin once removed Gordon & Julie 2nd cousin Gordon & Matt 2nd cousin once removed Gordon & Lyla 2nd cousin ${\textstyle \left\vert R-2\right\vert }$ times removed 1st cousin twice removed Sam & Julie 2nd cousin once removed Sam & Matt 3rd cousin Sam & Lyla 3rd cousin ${\textstyle \left\vert R-3\right\vert }$ times removed 1st cousin ${\textstyle \left\vert S-1\right\vert }$times removed 2nd cousin ${\displaystyle \left\vert S-2\right\vert }$times removed 3rd cousin ${\displaystyle \left\vert S-3\right\vert }$ times removed ${\textstyle (\min(S,R)-1)}$th cousin ${\displaystyle \left\vert R-S\right\vert }$ times removed
Basic family tree example
 Jason Beatrice James Helen Eugene Mary Nancy Joseph Julie Roger Gordon Laura Christina Matt Sam Lyla

The following is a list of less common cousin terms.

The terms cousin uncle/aunt and cousin niece/nephew or second uncle/aunt/nephew/niece are sometimes used to describe the direction of the removal of the relationship in Mennonite families. This term relates to a first cousin once removed. For additional removals grand/great is applied to the niece/nephews/uncle/aunt relationship. For example a second granduncle is a male first cousin twice removed that comes from a previous generation.

### Gender-based distinctions

A maternal cousin is a cousin that is related to the mother's side of the family, while a paternal cousin is a cousin that is related to the father's side of the family. Unlike all the other cousin relationships discussed thus far, this relationship is not necessarily reciprocal, as the maternal cousin of one person could be the paternal cousin of the other. In the example Basic Family Tree Julie is Joseph's maternal cousin and Joseph is Julie's paternal cousin.

Parallel and cross cousins on the other hand are reciprocal relationships. Parallel cousins are descended from same-sex siblings. Cousins that are related to same-sex siblings of their most recent common ancestor are parallel cousins. A parallel first cousin relationship exist when both the subject and relative are maternal cousins, or both are paternal cousins.

Cross cousins are descendants from opposite-sex siblings. A cross first cousin relationship exist when the subject and the relative are maternal cousins and paternal cousin to each other. In the example Basic Family Tree Joseph and Julie are both cross cousins.

### Multiplicities

Example family tree Chart
Double cousins
 Gary Glenda Jason Beatrice James Helen Eugene Mary Joseph Julie
Half cousins
 Jason Beatrice Anthony James Helen Charles Janet Joseph Lilian

Double cousins arise when two siblings of one family mate with two siblings of another family.[8] This may also be referred to as 'cousins on both sides.' The resulting children are related to each other through both of their parents and are thus doubly related. Double first cousins share both sets of grandparents. In the double cousins example family tree Joseph and Julie are double first cousins because each is related through their mother's family and also their father's family, the result of a brother and sister (Helen and Eugene) having married another brother and sister (James and Mary). For Joseph and Julie, each has a mother who is an aunt by blood of the other and a father who is an uncle by blood of the other.

Half cousins are descended from half siblings and would share 1 grandparent.[9] The children of two half siblings are first half cousins. If half siblings have children with another pair of half siblings, the resulting children would be double half first cousins. In the half cousins example family tree Joseph and Lilian are half cousins because their parents (Helen and Charles) are half-siblings, their grandmother (Beatrice) having remarried.

While there is no agreed upon term, it is possible for cousins to share three grandparents if a pair of half siblings had children with a pair of full siblings.[10][11]

### Non-blood relations

Step-cousins family tree example
 Jason Beatrice James Helen Eugene Corinda Colin Joseph Rachel

Step-cousins are either stepchildren of an individual's aunt or uncle, nieces and nephews of one's stepparent, or the children of one's parent's stepsibling.[citation needed] Cousins in law are the cousins of a person's spouse or the spouse of a person's cousin.[12] Neither of these relationships has consanguinity. In the Basic Family Tree example Joseph and Roger are both Cousin-in-laws. In the Step-cousins family tree example below Joseph and Rachel are step-cousins because Joseph's uncle (Eugene) has become Rachel's stepfather as a result of Rachel's mother (Corinda) having remarried Eugene.

## Consanguinity

Consanguinity is a measure of how closely you are related to another individual. It is measured by the coefficient of relationship. Below, when discussing the coefficient of relationship, we assume the subject and the relative are only related through the kinship term. A coefficient of one represents the relationship you have with yourself. Consanguinity decreases by half for every generations 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.

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

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 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 .

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

### Reproduction

Couples that possess higher than normal consanguinity, shared identical DNA and genetic material, have an increased chance of sharing genes for recessive traits.[13] Therefore, children of high consanguinity parents may have an increased risk of genetic disorders, particularly if their parents both carry a harmful recessive mutation. See inbreeding for more information.

Scientists through multiple studies have established a substantial and consistent positive correlation between the kinship of couples and the number of children and grandchildren they have. The 2008 deCODE study results show that couples related at the level of third cousins have the greatest number of offspring, with the greatest reproductive success observed for couples related at the level of third and fourth cousins.[14] This study provides the most comprehensive answer yet to the longstanding question of how kinship affects human fertility. The study result was somewhat counterintuitive from an evolutionary perspective because closely related parents have a higher probability of having offspring homozygous for deleterious recessive mutations, although closer parental kinship can also decrease the likelihood of immunological incompatibility between mother and offspring, for example in rhesus factor blood type.[15] The study confirmed that the offspring of first and second cousins died younger and reproduced less.[16]

### Cousin marriage

Cousin marriage is important in several anthropological theories which often differentiate between matriarchal and patriarchal parallel and cross cousins.

Currently about 10% and historically as high as 80% of all marriages are between first or second cousins.[17][18] Cousin marriages are often arranged.[17][18][19][20][21] Anthropologists believe it is used as a tool to strengthen the family, conserve its wealth, protect its cultural heritage, and retain the power structure of the family and its place in the community. Some groups encourage cousin marriage while others attach a strong social stigma to it. In some regions in the Middle East over half of all marriages are between first and second cousins. In some of the countries in this region this may exceed 70%.[22] Just outside this region it is often legal but infrequent. In other places it is legally prohibited and culturally equivalent to incest.[23][24] Supporters of cousin marriage often view the prohibition as discrimination,[25][26] while opponents cite the potential immorality.[27]

## References

1. ^ "Cousin". Brewer's Dictionary of Phrase & Fable. Chambers Harrap Publishers. 2013. 19.
2. ^ "Cousin-german definition and meaning | Collins English Dictionary". www.collinsdictionary.com. Retrieved 2018-02-26.
3. ^ a b "Definition of Cousin by Merriam-Webster". merriam-webster.com. Merriam-Webster.
4. A Dictionary of Genetics. Oxford University Press. 2013. 8. Missing or empty |title= (help)
5. ^ "Definition of cousin in English by Oxford Dictionaries". oxforddictionaries.com. Oxford University Press.
6. ^ a b "Genetic And Quantitative Aspects Of Genealogy – Types Of Collateral Relationships". Genetic-genealogy.co.uk. Retrieved 28 October 2014.
7. ^ "What is a First Cousin, Twice Removed?". Genealogy.com. Retrieved Sep 26, 2015.
8. ^ Dr. Barry Starr (2015-01-13). "Relatedness". Stanford at The Tech: Understanding Genetics.
9. ^ Jillynne Quinn (2014-01-09). "Relatedness". Stanford at The Tech: Understanding Genetics.
10. ^ http://www.genetic-genealogy.co.uk/Toc115570136.html
11. ^ http://deepfriedhoodsiecups.wordpress.com/2011/02/20/2202011-g4bb-world-of-tinker-toys/
12. ^ "cousin-in-law". Webster's Dictionary.
13. ^ The Conversation: What’s the genetic disease risk for children of related couples? Date: September 27, 2012. Source: Tiong Tan, Clinical Geneticist at Victorian Clinical Genetics Services and Researcher in Craniofacial Research, Murdoch Children's Research Institute.
14. ^ PubMed: An association between the kinship and fertility of human couples. Free Full Text. Date: 2008 Feb 8; Source: http://www.decode.com/publications/ deCODE Genetics.
15. ^ Science Daily: Third Cousins Have Greatest Number Of Offspring, Data From Iceland Shows. Date: February 8, 2008; Source: deCODE genetics.
16. ^ Nature: When kissing cousins are good for kids - A little inbreeding might boost fertility. By Heidi Ledford. Date: Published online 7 February 2008.
17. ^ a b Kershaw, Sarah (26 November 2009). "Shaking Off the Shame". The New York Times.
18. ^ a b
19. ^ Bittles, Alan H. (May 2001). A Background Summary of Consanguineous Marriage (PDF) (Technical report). Edith Cowan University.
20. ^ Bittles 1994, p. 567
21. ^ Bittles and Black 2009, Section 7
22. ^ Dr. Alan Bittles; Dr. Michael Black. "Global prevalence". consang.net.
23. ^ "The Surprising Truth About Cousins and Marriage". 14 February 2014.
24. ^ Paul, Diane B.; Spencer, Hamish G. (23 December 2008). ""It's Ok, We're Not Cousins by Blood": The Cousin Marriage Controversy in Historical Perspective". PLOS Biology. 6 (12): 2627–30. doi:10.1371/journal.pbio.0060320. PMC 2605922. PMID 19108607.
25. ^ "Final Thoughts". Cousin Couples. Retrieved 4 June 2016.
26. ^ Brandon Keim (23 December 2008). "Cousin Marriage OK by Science". Wired.
27. ^ Saletan, William (10 April 2002). "The Love That Dare Not Speak Its Surname" – via Slate.