Commonly, "cousin" refers to a "first cousin", people whose most recent common ancestor is a grandparent. A first cousin used to be known as a cousin-german, though this term is rarely used today.
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.
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. Various governmental entities have established systems for legal use that can precisely specify kinship with common ancestors any number of generations in the past. Common usage often eliminates the degrees and removals, and refers to people with common ancestry as simply "distant cousins" or "relatives".
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).
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.
The removal of the cousin relationship is the number of generations the cousins are apart. 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. For example, if the most recent common ancestor is 2 generations prior for one person and 3 generations prior for the other (one person's grandfather is the other person's great-grandfather) or the most recent common ancestor is 3 generations prior for one person and 4 generations prior for the other (one person's great-grandfather is the other person's great-great-grandfather) the cousins are separated by one generation and therefore once removed. Note that two people can be removed but be around the same age due to differences in birth dates of parents children and other relevant ancestors.
The degree of the cousin relationship is the number of generations prior to the parents before a most recent common ancestor is found. If the cousins are removed, the smaller number of generations to the most recent common ancestor is used to determine the degree of the cousin relationship. For example, if one of the cousins has to go back one generation beyond their parents (the grandparents) before finding the most recent common ancestor and the other has to go back one or more they are first cousins. If one had to go back two generations beyond the parents (great grandparents) and the other had to go back two or more they would be second cousins.
Double cousins arise when two siblings of one family mate with two siblings of another family. 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 and have twice the degree of consanguinity of ordinary first cousins.
Half cousins are descended from half siblings. 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 first half cousins.
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.
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 is either the paternal cousin on the father's side of the family or the maternal cousin on the mother's side of the family. Cross cousins are descendants from opposite-sex siblings. A cross first cousin is either the maternal cousin on the father's side of the family or the paternal cousin on the mother's side of the family.
Stepcousins 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. Cousins in law are the cousins of a person's spouse or the spouse of a person's cousin. Neither of these relationships have consanguinity.
A "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.
|If the relative's ->||Parent||Grandparent||Great-grandparent||Great-great-grandparent||Great-great-great-grandparent||Great-great-great-great-grandparent|
|Is the subject's
|Then the relative is the subject's ?|
||1st cousins||1st cousins once removed||1st cousins twice removed||1st cousins three times removed||1st cousins four times removed|
||1st cousins once removed||2nd cousins||2nd cousins once removed||2nd cousins twice removed||2nd cousins three times removed|
||1st cousins twice removed||2nd cousins once removed||3rd cousins||3rd cousins once removed||3rd cousins twice removed|
||1st cousins three times removed||2nd cousins twice removed||3rd cousins once removed||4th cousins||4th cousins once removed|
||1st cousins four times removed||2nd cousins three times removed||3rd cousins twice removed||4th cousins once removed||5th cousins|
Another visual chart used in determining the legal relationship between two people who share a common ancestor is a "canon law relationship chart".
The chart is the same as the previous chart with the exception that the chart is symmetrical and has been rotated 45° so that the common progenitor (most recent common ancestor) is placed at the peak. Since the graph is symmetrical the placement of either party in the relationship on either side of the graph is arbitrary. The graph does not distinguish between parents and children, or aunts/uncles and nieces/nephews. For these non symmetrical relationships the relationship of the person from the most recent generation is displayed (i.e. children and nieces/nephews).
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 over 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. 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.
Married couples that possess higher than normal consanguinity, shared identical DNA and genetic material, have an increased chance of sharing genes for recessive traits. The percentage of consanguinity between any two individuals decreases fourfold as the most recent common ancestor recedes one generation. First cousins have four times the consanguinity of second cousins, while first cousins once removed have half that of first cousins. Double first cousins share both sets of grandparents and have twice the degree of consanguinity of ordinary first cousins; genetically, they are as related as half-siblings. Children of these marriages may have an increased risk of genetic disorders, particularly if their parents both carry a harmful recessive mutation. 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.
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. This study provides the most comprehensive answer yet to the longstanding question of how kinship affects fertility in humans. 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. The study confirmed that the offspring of first and second cousins died younger and reproduced less.
The following is a list of less common cousin terms.
|Double cousin||Double cousins arise when two siblings of one family mate with two siblings of another family.||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-cousin||Half-cousins are the children of two half-siblings, and their respective spouses.||Joseph and Lilian are half cousins because their parents (Helen and Charles) are half-siblings, their grandmother (Beatrice) having remarried.|
|Stepcousin||Stepcousins are either stepchildren of an individual's aunt or uncle or nieces and nephews of one's stepparent.||Joseph and Rachel are stepcousins because Joseph's uncle (Eugene) has become Rachel's stepfather as a result of Rachel's mother (Corinda) having remarried Eugene.|
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