Cousin

Acousinis a relative that is the child of a parent's sibling; this is more specifically referred to as afirst cousin.
More generally, in thekinshipsystem used in the English-speaking world, a cousin is a type ofrelationshipin which relatives are two or moregenerationsaway from their most recent common ancestor. For this definition degrees and removals are used to further specify the relationship.
Degree measures how distant the relationship is from the most recent common ancestor(s). If the cousins do not come from the same generation, removal is specified, as removal measures the difference in generations between the two cousins.  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 ofthird-degree relative.
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 anancestorof the other, they do not share a parent (are notsiblings), and neither is a sibling of the other's parent (are not the other'saunt/unclenorniece/nephew).In the English system the cousin relationship is further detailed by the concepts ofdegreeandremoval.
Thedegreeis 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.Theremovalis 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.
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 exampleBasic family tree, Emma is David's maternal cousin and David is Emma's paternal cousin.
On the other hand,parallel and cross cousinsare 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 thebasic family treeexample, David and Emma are cross cousins.
Double cousins are relatives who are cousins from two different branches of the family tree. This occurs whensiblings, 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 theirparentsand are thus doubly related. Double first cousins share both sets ofgrandparents.
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 and would share one grandparent on each side.
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.
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 one's spouse or the spouse of one's cousin.
Consanguinityis a measure of how closely individuals are related to each other. It is measured by thecoefficient of relationship. 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 themost 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:(12)4+(12)4{\displaystyle \left({\tfrac {1}{2}}\right)^{4}+\left({\tfrac {1}{2}}\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 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 have children with another pair of monozygotic twins, the resulting double cousins would test as genetically similar as siblings.
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 oftenarranged.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 strongsocial stigmato it. In some regions in theMiddle East, more than half of all marriages are between first or second cousins (in 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 islegally prohibitedand culturally equivalent toincest.Supporters of cousin marriage often view the prohibition asdiscrimination,while opponents claim potentialimmorality.
Couples that are closely related have an increased chance of sharing genes, including mutations that occurred in their family tree. If the mutation is arecessive trait, 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 ofrecessive genetic disorders.
Research has indicated that related couples have more children - a study of Icelandic couples concluded that those related with consanguinity equivalent to that of third cousins have the greatestreproductive success.This seems counterintuitive as closely related parents have a higher probability of having offspring that areunfit, yet closer kinship can also decrease the likelihood of immunological incompatibility during pregnancy.
This risks of inbreeding - particularly through common practice of cousin marriage - extend beyond the immediate descendants of those involved. Sustained cousin marriage within a population group over generations eventually leads toinbreedingat the population level and the rise of genetic disorders that affects the entire population, not just those in cousin marriages.Increasedgenetic literacywithin a population can lead to a reduction in cousin marriage.