Cousin

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Most generally, in the lineal kinship system used in the English-speaking world, a cousin is a type of familial relationship in which two relatives are two or more familial generations away from their most recent common ancestor. Commonly, "cousin" refers to a first cousin – a relative of the same generation whose
most recent common ancestor In biology and genetic genealogy, the most recent common ancestor (MRCA), also known as the last common ancestor (LCA) or concestor, of a set of organisms is the most recent individual from which all the organisms of the set are Common descent, ...
with the subject is a
grandparent Grandparents, individually known as grandmother and grandfather, are the parents of a person's father or mother – paternal or maternal. Every sexually-reproducing living organism who is not a genetic chimera has a maximum of four genetics, ge ...
. Degrees and removals are separate measures used to more precisely describe the relationship between cousins. ''Degree'' measures the separation, in generations, from the most recent common ancestor(s) to a parent of one of the cousins (whichever is closest), while ''removal'' measures the difference in generations between the cousins themselves, relative to their most recent common ancestor(s). To illustrate usage, a second cousin is a cousin with a ''degree'' of two; there are three (not two) generations from the common ancestor(s). When the degree is not specified, first cousin is assumed. A cousin "once removed" is a cousin with one removal. When the removal is not specified, no removal is assumed. 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.

# Basic definitions

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 An ancestor, also known as a forefather, fore-elder or a forebear, is a parent or (Recursion, recursively) the parent of an antecedent (i.e., a grandparent, great-grandparent, great-great-grandparent and so forth). ''Ancestor'' is "any person ...
of the other, they do not share a parent (are not
sibling A sibling is a relative that shares at least one parent with the subject. A male Male (Mars symbol, symbol: ♂) is the sex of an organism that produces the gamete (sex cell) known as sperm, which fuses with the larger female gamete, or ovum ...
s), and neither is a sibling of the other's parent (are not the other's
aunt An aunt is a woman who is a sibling of a parent or married to a sibling of a parent. Aunts who are consanguineous, related by birth are Second-degree relative, second-degree relatives. Known alternate terms include auntie or aunty. Children in ...
/
uncle An uncle is usually defined as a male kinship, relative who is a sibling of a parent or married to a sibling of a parent. Uncles who are consanguineous, related by birth are second-degree relatives. The female counterpart of an uncle is an aunt, a ...
nor niece/nephew). 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. The ''removal'' is the difference between the number of generations from each cousin to the common ancestor. Two people can be removed but be around the same age due to differences in birth dates of parents, children, and other relevant ancestors. To illustrate these concepts the following table is provided. This table identifies the degree and removal of cousin relationship between two people using their most recent common ancestor as the reference point and demonstrates it in the basic family tree example.

* The terms ''full cousin'' and ''cousin-german'' are used to specify a first cousin with no removals. * The terms ''cousin-uncle/aunt'' and ''cousin-niece/nephew'' are sometimes used to describe the direction of the removal of the relationship, especially in Mennonite, Indian, and Pakistani 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. 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 A kiss is the touch or pressing of one's lips against another person or an object. Cultural connotations of kissing vary widely. Depending on the culture and context, a kiss can express sentiments of love Love encompasses a range of s ...
.

## 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. 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, Emma is David's maternal cousin and David is Emma's paternal cousin. Parallel and cross cousins on the other hand are reciprocal relationships. Parallel cousins are descended from same-sex siblings. A parallel first cousin relationship exists 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 exists when the subject and the relative are maternal cousin and paternal cousin to each other. In the basic family tree example, David and Emma are cross cousins.

## Multiplicities

Double cousins are relatives that are cousins from two different branches of the family tree. This occurs when
sibling A sibling is a relative that shares at least one parent with the subject. A male Male (Mars symbol, symbol: ♂) is the sex of an organism that produces the gamete (sex cell) known as sperm, which fuses with the larger female gamete, or ovum ...
s, respectively, reproduce with different siblings from another family. This may also be referred to as "cousins on both sides". The resulting children are related to each other through both their
parent A parent is a caregiver of the offspring in their own species. In humans, a parent is the caretaker of a child (where "child" refers to offspring, not necessarily age). A ''biological parent'' is a person whose gamete resulted in a child, a male t ...
s and are thus doubly related. Double first cousins share both sets of
grandparent Grandparents, individually known as grandmother and grandfather, are the parents of a person's father or mother – paternal or maternal. Every sexually-reproducing living organism who is not a genetic chimera has a maximum of four genetics, ge ...
s. Half cousins are descended from half siblings and would share one grandparent. 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. 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.

## Non-blood relations

Step-cousins are either stepchildren of an individual's aunt or uncle, nieces and nephews of one's step-parent, or the children of one's parent's step-sibling. A cousin-in-law is the cousin of a person's spouse or the spouse of a person's cousin. In the Basic family tree example David and Edward are cousins-in-law. None of these relationships have consanguinity.

# Consanguinity

Consanguinity is a measure of how closely individuals are related to each other. It is measured by the
coefficient of relationship The coefficient of relationship is a measure of the degree of consanguinity (or biological relationship) between two individuals. The term coefficient of relationship was defined by Sewall Wright in 1922, and was derived from his definition of th ...
. 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 In biology and genetic genealogy, the most recent common ancestor (MRCA), also known as the last common ancestor (LCA) or concestor, of a set of organisms is the most recent individual from which all the organisms of the set are Common descent, ...
, 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: $\left\left(\tfrac\right\right)^4 + \left\left(\tfrac\right\right)^4$; 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. Half cousins have half the
consanguinity Consanguinity ("blood relation", from Latin '':wikt: consanguinitas, consanguinitas'') is the characteristic of having a kinship with another person (being descended from a common ancestor). Many jurisdictions have laws prohibiting people who ar ...
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. 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 are closely related have an increased chance of sharing genes, including mutations that occurred in their family tree. If the mutation is a
recessive trait In genetics, dominance is the phenomenon of one variant (allele) of a gene on a chromosome masking or overriding the Phenotype, effect of a different variant of the same gene on Homologous chromosome, the other copy of the chromosome. The first ...
, it will not reveal itself unless both father and mother share it. Due to the risk that the trait is harmful, children of high-consanguinity parents have an increased risk of recessive genetic disorders. See
inbreeding Inbreeding is the production of offspring from the mating or breeding of individuals or organisms that are closely genetic distance, related genetically. By analogy, the term is used in human reproduction, but more commonly refers to the geneti ...
for more information. Closely related couples have more children. Couples related with consanguinity equivalent to that of third cousins have the greatest reproductive success. This seems counterintuitive as closely related parents have a higher probability of having offspring that are unfit, yet closer kinship can also decrease the likelihood of immunological incompatibility during pregnancy.

## 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. Cousin marriages are often arranged. 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 Social stigma is the disapproval of, or discrimination against, an individual or group based on perceived characteristics that serve to distinguish them from other members of a society. Social stigmas are commonly related to culture, gender, rac ...
to it. In some regions in the
Middle East The Middle East ( ar, الشرق الأوسط, ISO 233: ) is a geopolitical region commonly encompassing Arabian Peninsula, Arabia (including the Arabian Peninsula and Bahrain), Anatolia, Asia Minor (Asian part of Turkey except Hatay Pro ...
, more than half of all marriages are between first or second cousins (some of the countries in this region this may exceed 70%). Just outside this region, it is often legal but infrequent. Many cultures have encouraged specifically cross-cousin marriages. In other places, it is legally prohibited and culturally equivalent to
incest Incest ( ) is human sexual activity between family members or close kinship, relatives. This typically includes sexual activity between people in consanguinity (blood relations), and sometimes those related by Affinity (law), affinity (marriage ...
. Supporters of cousin marriage often view the prohibition as
discrimination Discrimination is the act of making unjustified distinctions between people based on the groups, classes, or other categories to which they belong or are perceived to belong. People may be discriminated on the basis of Racial discrimination, r ...
, while opponents claim potential
immorality Immorality is the violation of morality, moral laws, Norm (social), norms or standards. It refers to an agent doing or thinking something they know or believe to be wrong. Immorality is normally applied to people or actions, or in a broader sense ...
and cite the increased rate of
birth defects A birth defect, also known as a congenital disorder, is an abnormal condition that is present at childbirth, birth regardless of its cause. Birth defects may result in disability, disabilities that may be physical disability, physical, intellect ...
in children of cousin marriages.

*
Collateral descendant A lineal descendant, in legal usage, is a blood Kinship, relative in the direct line of Kinship, descent – the children, grandchildren, great-grandchildren, etc. of a person. In a legal procedure sense, lineal descent refers to the acquisition ...
*
Consanguinity Consanguinity ("blood relation", from Latin '':wikt: consanguinitas, consanguinitas'') is the characteristic of having a kinship with another person (being descended from a common ancestor). Many jurisdictions have laws prohibiting people who ar ...
*
Cousin marriage A cousin marriage is a marriage where the spouses are cousins (i.e. people with common grandparents or people who share other fairly recent ancestors). The practice was common in earlier times, and continues to be common in some societies toda ...
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Family Family (from la, familia) is a Social group, group of people related either by consanguinity (by recognized birth) or Affinity (law), affinity (by marriage or other relationship). The purpose of the family is to maintain the well-being of its ...
*
Sibling A sibling is a relative that shares at least one parent with the subject. A male Male (Mars symbol, symbol: ♂) is the sex of an organism that produces the gamete (sex cell) known as sperm, which fuses with the larger female gamete, or ovum ...
*
Second-degree relative A second-degree relative (SDR) is someone who shares 25% of a person's genes. It includes uncles, aunts, nephews, nieces, grandparents, grandchildren, half-siblings, and double cousins. See also *Family *First-degree relative *Third-degree relati ...