Commonly "cousin" refers to a "first cousin", a relative whose most recent common ancestor with the subject is a grandparent. More 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.
Degrees and removals are used to more precisely describe the relationship between cousins. Degree measures the separation, in generations, from the most recent common ancestor to one of the cousins (whichever is closest), while removal measures the difference in generations between the cousins themselves. To illustrate usage, a "second cousin" is a cousin with a degree of two. When the degree is not specified first cousin is assumed. A cousin that is "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 law, a first cousin is a type of third-degree relative.
|Basic family tree|
|The relationship between every solid shaded box and a similar one on the other branch of the tree is that of a cousin. The removal is the number of rows the relatives are separated by. The degree of the relationship is that of the relative with the lowest order. The rules are the same for cousin-in-laws, except they exist between shaded solid lines and shaded dotted lines.|
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 of the other, they do not share a parent (are not siblings), and neither is a sibling of a common ancestor (aunts/uncles and nieces/nephews). 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 example family tree.
|Separation in generations to ancestor R->||2||3||4|
|S?||Relationship to ancestor||Grandparent||Great-grandparent||Great-great-grandparent|
|2||Grandparent||1st cousin||1st cousin once removed||1st cousin twice removed|
|David & Emma||David & George||David & Imogen|
|3||Great-grandparent||1st cousin once removed||2nd cousin||2nd cousin once removed|
|Frank & Emma||Frank & George||Frank & Imogen|
|4||Great-great-grandparent||1st cousin twice removed||2nd cousin once removed||3rd cousin|
|Harry & Emma||Harry & George||Harry & Imogen|
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. Cousins that are related to same-sex siblings of their most recent common ancestor are parallel cousins. 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 cousins and paternal cousin to each other. In the example Basic family tree David and Emma are both cross cousins.
|Ewan and Fannie are double first cousins because they share both sets of grandparents as they are cousins through both parents. They are cousins through the siblings Claude and Darrell as well as the siblings Colleen and Dorothea.|
Double cousins are relatives that are cousins from two different branches of the family tree. This occurs when siblings, 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 parents and are thus doubly related. Double first cousins share both sets of grandparents.
|David and Esme are half cousins as they share only one grandparent (Agatha) because they are related through half-siblings (Betty and Cyrus).|
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.
|David and Evangeline are step-cousins because David's uncle (Charles) is now Evangeline's stepfather, Evangeline's mother (Corinda) having married Charles.|
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. 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 both cousins-in-law. None of these relationships have consanguinity.
Consanguinity is a measure of how closely individuals are related to each other. It is measured by the coefficient 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 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 (), 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.
Offspring of first and second cousins die younger and reproduce less. 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 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 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 counter-intuitive 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 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 to it. In some regions in the Middle East more than half of all marriages are between first and 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 is legally prohibited and culturally equivalent to incest. Supporters of cousin marriage often view the prohibition as discrimination, while opponents cite the potential immorality and the increased rate of birth defects in children of cousin marriages.
They seemed to treasure genetic relations in a way I had not encountered before, using such relational designations as "cousin-uncle." They spoke of Mennonite names, Mennonite food, Low German, Russian immigration.